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
    },
```

lighthouse/src/main.rs

@@ -115,7 +115,7 @@ fn main() {
                 .long("testnet")
                 .value_name("testnet")
                 .help("Name of network lighthouse will connect to")
-                .possible_values(&["medalla", "altona", "spadina", "zinken"])
+                .possible_values(&["medalla", "altona", "spadina", "zinken", "mainnet"])
                 .conflicts_with("testnet-dir")
                 .takes_value(true)
                 .global(true)
@@ -265,15 +265,22 @@ fn run<E: EthSpec>(
 
     warn!(
         log,
-        "Ethereum 2.0 is pre-release. This software is experimental."
+        "Ethereum 2.0 is pre-release. This software is experimental"
     );
     info!(log, "Lighthouse started"; "version" => VERSION);
     info!(
         log,
         "Configured for testnet";
-        "name" => testnet_name
+        "name" => &testnet_name
     );
 
+    if testnet_name == "mainnet" {
+        warn!(
+            log,
+            "The mainnet specification is being used. This not recommended (yet)."
+        )
+    }
+
     match matches.subcommand() {
         ("beacon_node", Some(matches)) => {
             let context = environment.core_context();