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>

validator_client/src/fork_service.rs

@@ -1,7 +1,7 @@
 use environment::RuntimeContext;
+use eth2::{types::StateId, BeaconNodeHttpClient};
 use futures::StreamExt;
 use parking_lot::RwLock;
-use remote_beacon_node::RemoteBeaconNode;
 use slog::{debug, trace};
 use slot_clock::SlotClock;
 use std::ops::Deref;
@@ -16,7 +16,7 @@ const TIME_DELAY_FROM_SLOT: Duration = Duration::from_millis(80);
 pub struct ForkServiceBuilder<T, E: EthSpec> {
     fork: Option<Fork>,
     slot_clock: Option<T>,
-    beacon_node: Option<RemoteBeaconNode<E>>,
+    beacon_node: Option<BeaconNodeHttpClient>,
     context: Option<RuntimeContext<E>>,
 }
 
@@ -35,7 +35,7 @@ impl<T: SlotClock + 'static, E: EthSpec> ForkServiceBuilder<T, E> {
         self
     }
 
-    pub fn beacon_node(mut self, beacon_node: RemoteBeaconNode<E>) -> Self {
+    pub fn beacon_node(mut self, beacon_node: BeaconNodeHttpClient) -> Self {
         self.beacon_node = Some(beacon_node);
         self
     }
@@ -66,7 +66,7 @@ impl<T: SlotClock + 'static, E: EthSpec> ForkServiceBuilder<T, E> {
 /// Helper to minimise `Arc` usage.
 pub struct Inner<T, E: EthSpec> {
     fork: RwLock<Option<Fork>>,
-    beacon_node: RemoteBeaconNode<E>,
+    beacon_node: BeaconNodeHttpClient,
     context: RuntimeContext<E>,
     slot_clock: T,
 }
@@ -141,9 +141,7 @@ impl<T: SlotClock + 'static, E: EthSpec> ForkService<T, E> {
         let fork = self
             .inner
             .beacon_node
-            .http
-            .beacon()
-            .get_fork()
+            .get_beacon_states_fork(StateId::Head)
             .await
             .map_err(|e| {
                 trace!(
@@ -151,7 +149,15 @@ impl<T: SlotClock + 'static, E: EthSpec> ForkService<T, E> {
                     "Fork update failed";
                     "error" => format!("Error retrieving fork: {:?}", e)
                 )
-            })?;
+            })?
+            .ok_or_else(|| {
+                trace!(
+                    log,
+                    "Fork update failed";
+                    "error" => "The beacon head fork is unknown"
+                )
+            })?
+            .data;
 
         if self.fork.read().as_ref() != Some(&fork) {
             *(self.fork.write()) = Some(fork);