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/router/processor.rs

@@ -5,7 +5,7 @@ use crate::service::NetworkMessage;
 use crate::sync::SyncMessage;
 use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
 use eth2_libp2p::rpc::*;
-use eth2_libp2p::{MessageId, NetworkGlobals, PeerId, PeerRequestId, Request, Response};
+use eth2_libp2p::{Client, MessageId, NetworkGlobals, PeerId, PeerRequestId, Request, Response};
 use slog::{debug, error, o, trace, warn};
 use std::cmp;
 use std::sync::Arc;
@@ -211,6 +211,7 @@ impl<T: BeaconChainTypes> Processor<T> {
                 peer_id,
                 request_id: id,
                 beacon_block,
+                seen_timestamp: timestamp_now(),
             });
         } else {
             debug!(
@@ -229,11 +230,13 @@ impl<T: BeaconChainTypes> Processor<T> {
         &mut self,
         message_id: MessageId,
         peer_id: PeerId,
+        peer_client: Client,
         block: Box<SignedBeaconBlock<T::EthSpec>>,
     ) {
         self.send_beacon_processor_work(BeaconWorkEvent::gossip_beacon_block(
             message_id,
             peer_id,
+            peer_client,
             block,
             timestamp_now(),
         ))