Implement standard eth2.0 API (#1569)

- Resolves #1550
- Resolves #824
- Resolves #825
- Resolves #1131
- Resolves #1411
- Resolves #1256
- Resolve #1177

- Includes the `ShufflingId` struct initially defined in #1492. That PR is now closed and the changes are included here, with significant bug fixes.
- Implement the https://github.com/ethereum/eth2.0-APIs in a new `http_api` crate using `warp`. This replaces the `rest_api` crate.
- Add a new `common/eth2` crate which provides a wrapper around `reqwest`, providing the HTTP client that is used by the validator client and for testing. This replaces the `common/remote_beacon_node` crate.
- Create a `http_metrics` crate which is a dedicated server for Prometheus metrics (they are no longer served on the same port as the REST API). We now have flags for `--metrics`, `--metrics-address`, etc.
- Allow the `subnet_id` to be an optional parameter for `VerifiedUnaggregatedAttestation::verify`. This means it does not need to be provided unnecessarily by the validator client.
- Move `fn map_attestation_committee` in `mod beacon_chain::attestation_verification` to a new `fn with_committee_cache` on the `BeaconChain` so the same cache can be used for obtaining validator duties.
- Add some other helpers to `BeaconChain` to assist with common API duties (e.g., `block_root_at_slot`, `head_beacon_block_root`).
- Change the `NaiveAggregationPool` so it can index attestations by `hash_tree_root(attestation.data)`. This is a requirement of the API.
- Add functions to `BeaconChainHarness` to allow it to create slashings and exits.
- Allow for `eth1::Eth1NetworkId` to go to/from a `String`.
- Add functions to the `OperationPool` to allow getting all objects in the pool.
- Add function to `BeaconState` to check if a committee cache is initialized.
- Fix bug where `seconds_per_eth1_block` was not transferring over from `YamlConfig` to `ChainSpec`.
- Add the `deposit_contract_address` to `YamlConfig` and `ChainSpec`. We needed to be able to return it in an API response.
- Change some uses of serde `serialize_with` and `deserialize_with` to a single use of `with` (code quality).
- Impl `Display` and `FromStr` for several BLS fields.
- Check for clock discrepancy when VC polls BN for sync state (with +/- 1 slot tolerance). This is not intended to be comprehensive, it was just easy to do.

- See #1434 for a per-endpoint overview.
- Seeking clarity here: https://github.com/ethereum/eth2.0-APIs/issues/75

- [x] Add docs for prom port to close #1256
- [x] Follow up on this #1177
- [x] ~~Follow up with #1424~~ Will fix in future PR.
- [x] Follow up with #1411
- [x] ~~Follow up with  #1260~~ Will fix in future PR.
- [x] Add quotes to all integers.
- [x] Remove `rest_types`
- [x] Address missing beacon block error. (#1629)
- [x] ~~Add tests for lighthouse/peers endpoints~~ Wontfix
- [x] ~~Follow up with validator status proposal~~ Tracked in #1434
- [x] Unify graffiti structs
- [x] ~~Start server when waiting for genesis?~~ Will fix in future PR.
- [x] TODO in http_api tests
- [x] Move lighthouse endpoints off /eth/v1
- [x] Update docs to link to standard

- ~~Blocked on #1586~~

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/eth1/src/http.rs

@@ -39,19 +39,34 @@ pub enum Eth1NetworkId {
     Custom(u64),
 }
 
+impl Into<u64> for Eth1NetworkId {
+    fn into(self) -> u64 {
+        match self {
+            Eth1NetworkId::Mainnet => 1,
+            Eth1NetworkId::Goerli => 5,
+            Eth1NetworkId::Custom(id) => id,
+        }
+    }
+}
+
+impl From<u64> for Eth1NetworkId {
+    fn from(id: u64) -> Self {
+        let into = |x: Eth1NetworkId| -> u64 { x.into() };
+        match id {
+            id if id == into(Eth1NetworkId::Mainnet) => Eth1NetworkId::Mainnet,
+            id if id == into(Eth1NetworkId::Goerli) => Eth1NetworkId::Goerli,
+            id => Eth1NetworkId::Custom(id),
+        }
+    }
+}
+
 impl FromStr for Eth1NetworkId {
     type Err = String;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        match s {
-            "1" => Ok(Eth1NetworkId::Mainnet),
-            "5" => Ok(Eth1NetworkId::Goerli),
-            custom => {
-                let network_id = u64::from_str_radix(custom, 10)
-                    .map_err(|e| format!("Failed to parse eth1 network id {}", e))?;
-                Ok(Eth1NetworkId::Custom(network_id))
-            }
-        }
+        u64::from_str_radix(s, 10)
+            .map(Into::into)
+            .map_err(|e| format!("Failed to parse eth1 network id {}", e))
     }
 }