Add BlockTimesCache to allow additional block delay metrics (#2546)

## Issue Addressed

Closes #2528

## Proposed Changes

- Add `BlockTimesCache` to provide block timing information to `BeaconChain`. This allows additional metrics to be calculated for blocks that are set as head too late.
- Thread the `seen_timestamp` of blocks received from RPC responses (except blocks from syncing) through to the sync manager, similar to what is done for blocks from gossip.

## Additional Info

This provides the following additional metrics:
- `BEACON_BLOCK_OBSERVED_SLOT_START_DELAY_TIME`
  - The delay between the start of the slot and when the block was first observed.
- `BEACON_BLOCK_IMPORTED_OBSERVED_DELAY_TIME`
   - The delay between when the block was first observed and when the block was imported.
- `BEACON_BLOCK_HEAD_IMPORTED_DELAY_TIME`
  - The delay between when the block was imported and when the block was set as head.

The metric `BEACON_BLOCK_IMPORTED_SLOT_START_DELAY_TIME` was removed.

A log is produced when a block is set as head too late, e.g.:
```
Aug 27 03:46:39.006 DEBG Delayed head block                      set_as_head_delay: Some(21.731066ms), imported_delay: Some(119.929934ms), observed_delay: Some(3.864596988s), block_delay: 4.006257988s, slot: 1931331, proposer_index: 24294, block_root: 0x937602c89d3143afa89088a44bdf4b4d0d760dad082abacb229495c048648a9e, service: beacon
```

beacon_node/network/src/beacon_processor/mod.rs

@@ -42,7 +42,7 @@ use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::{BeaconChain, BeaconChainTypes, BlockError, GossipVerifiedBlock};
 use eth2_libp2p::{
     rpc::{BlocksByRangeRequest, BlocksByRootRequest, StatusMessage},
-    MessageId, NetworkGlobals, PeerId, PeerRequestId,
+    Client, MessageId, NetworkGlobals, PeerId, PeerRequestId,
 };
 use futures::stream::{Stream, StreamExt};
 use futures::task::Poll;
@@ -341,6 +341,7 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
     pub fn gossip_beacon_block(
         message_id: MessageId,
         peer_id: PeerId,
+        peer_client: Client,
         block: Box<SignedBeaconBlock<T::EthSpec>>,
         seen_timestamp: Duration,
     ) -> Self {
@@ -349,6 +350,7 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
             work: Work::GossipBlock {
                 message_id,
                 peer_id,
+                peer_client,
                 block,
                 seen_timestamp,
             },
@@ -602,6 +604,7 @@ pub enum Work<T: BeaconChainTypes> {
     GossipBlock {
         message_id: MessageId,
         peer_id: PeerId,
+        peer_client: Client,
         block: Box<SignedBeaconBlock<T::EthSpec>>,
         seen_timestamp: Duration,
     },
@@ -1362,11 +1365,13 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                     Work::GossipBlock {
                         message_id,
                         peer_id,
+                        peer_client,
                         block,
                         seen_timestamp,
                     } => worker.process_gossip_block(
                         message_id,
                         peer_id,
+                        peer_client,
                         *block,
                         work_reprocessing_tx,
                         seen_timestamp,