Add --testnet mainnet and start HTTP server before genesis (#1862)

## Issue Addressed

NA

## Proposed Changes

- Adds support for `--testnet mainnet`
- Start HTTP server prior to genesis

## Additional Info

**Note: This is an incomplete work-in-progress. Use Lighthouse for mainnet at your own risk.**

With this PR, you can check the deposits:

```bash
lighthouse --testnet mainnet bn --http
```
```bash
curl localhost:5052/lighthouse/eth1/deposit_cache | jq
```

```json
{
  "data": [
    {
      "deposit_data": {
        "pubkey": "0x854980aa9bf2e84723e1fa6ef682e3537257984cc9cb1daea2ce6b268084b414f0bb43206e9fa6fd7a202357d6eb2b0d",
        "withdrawal_credentials": "0x00cacf703c658b802d55baa2a5c1777500ef5051fc084330d2761bcb6ab6182b",
        "amount": "32000000000",
        "signature": "0xace226cdfd9da6b1d827c3a6ab93f91f53e8e090eb6ca5ee7c7c5fe3acc75558240ca9291684a2a7af5cac67f0558d1109cc95309f5cdf8c125185ec9dcd22635f900d791316924aed7c40cff2ffccdac0d44cf496853db678c8c53745b3545b"
      },
      "block_number": 3492981,
      "index": 0,
      "signature_is_valid": true
    },
    {
      "deposit_data": {
        "pubkey": "0x93da03a71bc4ed163c2f91c8a54ea3ba2461383dd615388fd494670f8ce571b46e698fc8d04b49e4a8ffe653f581806b",
        "withdrawal_credentials": "0x006ebfbb7c8269a78018c8b810492979561d0404d74ba9c234650baa7524dcc4",
        "amount": "32000000000",
        "signature": "0x8d1f4a1683f798a76effcc6e2cdb8c3eed5a79123d201c5ecd4ab91f768a03c30885455b8a952aeec3c02110457f97ae0a60724187b6d4129d7c352f2e1ac19b4210daacd892fe4629ad3260ce2911dceae3890b04ed28267b2d8cb831f6a92d"
      },
      "block_number": 3493427,
      "index": 1,
      "signature_is_valid": true
    },
```

common/eth2_testnet_config/built_in_testnet_configs/mainnet/boot_enr.yaml

@@ -0,0 +1 @@
+[]

common/eth2_testnet_config/built_in_testnet_configs/mainnet/config.yaml

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+# Mainnet preset
+
+CONFIG_NAME: "mainnet"
+
+# Misc
+# ---------------------------------------------------------------
+# 2**6 (= 64)
+MAX_COMMITTEES_PER_SLOT: 64
+# 2**7 (= 128)
+TARGET_COMMITTEE_SIZE: 128
+# 2**11 (= 2,048)
+MAX_VALIDATORS_PER_COMMITTEE: 2048
+# 2**2 (= 4)
+MIN_PER_EPOCH_CHURN_LIMIT: 4
+# 2**16 (= 65,536)
+CHURN_LIMIT_QUOTIENT: 65536
+# See issue 563
+SHUFFLE_ROUND_COUNT: 90
+# `2**14` (= 16,384)
+MIN_GENESIS_ACTIVE_VALIDATOR_COUNT: 16384
+# Dec 1, 2020, 12pm UTC
+MIN_GENESIS_TIME: 1606824000
+# 4
+HYSTERESIS_QUOTIENT: 4
+# 1 (minus 0.25)
+HYSTERESIS_DOWNWARD_MULTIPLIER: 1
+# 5 (plus 1.25)
+HYSTERESIS_UPWARD_MULTIPLIER: 5
+
+
+# Fork Choice
+# ---------------------------------------------------------------
+# 2**3 (= 8)
+SAFE_SLOTS_TO_UPDATE_JUSTIFIED: 8
+
+
+# Validator
+# ---------------------------------------------------------------
+# 2**11 (= 2,048)
+ETH1_FOLLOW_DISTANCE: 2048
+# 2**4 (= 16)
+TARGET_AGGREGATORS_PER_COMMITTEE: 16
+# 2**0 (= 1)
+RANDOM_SUBNETS_PER_VALIDATOR: 1
+# 2**8 (= 256)
+EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION: 256
+# 14 (estimate from Eth1 mainnet)
+SECONDS_PER_ETH1_BLOCK: 14
+
+
+# Deposit contract
+# ---------------------------------------------------------------
+# Ethereum PoW Mainnet
+DEPOSIT_CHAIN_ID: 1
+DEPOSIT_NETWORK_ID: 1
+# **TBD**
+DEPOSIT_CONTRACT_ADDRESS: 0x00000000219ab540356cBB839Cbe05303d7705Fa
+
+
+# Gwei values
+# ---------------------------------------------------------------
+# 2**0 * 10**9 (= 1,000,000,000) Gwei
+MIN_DEPOSIT_AMOUNT: 1000000000
+# 2**5 * 10**9 (= 32,000,000,000) Gwei
+MAX_EFFECTIVE_BALANCE: 32000000000
+# 2**4 * 10**9 (= 16,000,000,000) Gwei
+EJECTION_BALANCE: 16000000000
+# 2**0 * 10**9 (= 1,000,000,000) Gwei
+EFFECTIVE_BALANCE_INCREMENT: 1000000000
+
+
+# Initial values
+# ---------------------------------------------------------------
+# Mainnet initial fork version, recommend altering for testnets
+GENESIS_FORK_VERSION: 0x00000000
+BLS_WITHDRAWAL_PREFIX: 0x00
+
+
+# Time parameters
+# ---------------------------------------------------------------
+# 604800 seconds (7 days)
+GENESIS_DELAY: 604800
+# 12 seconds
+SECONDS_PER_SLOT: 12
+# 2**0 (= 1) slots 12 seconds
+MIN_ATTESTATION_INCLUSION_DELAY: 1
+# 2**5 (= 32) slots 6.4 minutes
+SLOTS_PER_EPOCH: 32
+# 2**0 (= 1) epochs 6.4 minutes
+MIN_SEED_LOOKAHEAD: 1
+# 2**2 (= 4) epochs 25.6 minutes
+MAX_SEED_LOOKAHEAD: 4
+# 2**6 (= 64) epochs ~6.8 hours
+EPOCHS_PER_ETH1_VOTING_PERIOD: 64
+# 2**13 (= 8,192) slots ~13 hours
+SLOTS_PER_HISTORICAL_ROOT: 8192
+# 2**8 (= 256) epochs ~27 hours
+MIN_VALIDATOR_WITHDRAWABILITY_DELAY: 256
+# 2**8 (= 256) epochs ~27 hours
+SHARD_COMMITTEE_PERIOD: 256
+# 2**2 (= 4) epochs 25.6 minutes
+MIN_EPOCHS_TO_INACTIVITY_PENALTY: 4
+
+
+# State vector lengths
+# ---------------------------------------------------------------
+# 2**16 (= 65,536) epochs ~0.8 years
+EPOCHS_PER_HISTORICAL_VECTOR: 65536
+# 2**13 (= 8,192) epochs ~36 days
+EPOCHS_PER_SLASHINGS_VECTOR: 8192
+# 2**24 (= 16,777,216) historical roots, ~26,131 years
+HISTORICAL_ROOTS_LIMIT: 16777216
+# 2**40 (= 1,099,511,627,776) validator spots
+VALIDATOR_REGISTRY_LIMIT: 1099511627776
+
+
+# Reward and penalty quotients
+# ---------------------------------------------------------------
+# 2**6 (= 64)
+BASE_REWARD_FACTOR: 64
+# 2**9 (= 512)
+WHISTLEBLOWER_REWARD_QUOTIENT: 512
+# 2**3 (= 8)
+PROPOSER_REWARD_QUOTIENT: 8
+# 2**26 (= 67,108,864)
+INACTIVITY_PENALTY_QUOTIENT: 67108864
+# 2**7 (= 128) (lower safety margin at Phase 0 genesis)
+MIN_SLASHING_PENALTY_QUOTIENT: 128
+# 1 (lower safety margin at Phase 0 genesis)
+PROPORTIONAL_SLASHING_MULTIPLIER: 1
+
+
+# Max operations per block
+# ---------------------------------------------------------------
+# 2**4 (= 16)
+MAX_PROPOSER_SLASHINGS: 16
+# 2**1 (= 2)
+MAX_ATTESTER_SLASHINGS: 2
+# 2**7 (= 128)
+MAX_ATTESTATIONS: 128
+# 2**4 (= 16)
+MAX_DEPOSITS: 16
+# 2**4 (= 16)
+MAX_VOLUNTARY_EXITS: 16
+
+
+# Signature domains
+# ---------------------------------------------------------------
+DOMAIN_BEACON_PROPOSER: 0x00000000
+DOMAIN_BEACON_ATTESTER: 0x01000000
+DOMAIN_RANDAO: 0x02000000
+DOMAIN_DEPOSIT: 0x03000000
+DOMAIN_VOLUNTARY_EXIT: 0x04000000
+DOMAIN_SELECTION_PROOF: 0x05000000
+DOMAIN_AGGREGATE_AND_PROOF: 0x06000000

common/eth2_testnet_config/built_in_testnet_configs/mainnet/deploy_block.txt

@@ -0,0 +1 @@
+11184524

common/eth2_testnet_config/built_in_testnet_configs/mainnet/deposit_contract.txt

@@ -0,0 +1 @@
+0x00000000219ab540356cBB839Cbe05303d7705Fa