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/http_api/src/lib.rs

@@ -2475,6 +2475,9 @@ pub fn serve<T: BeaconChainTypes>(
                                 api_types::EventTopic::ContributionAndProof => {
                                     event_handler.subscribe_contributions()
                                 }
+                                api_types::EventTopic::LateHead => {
+                                    event_handler.subscribe_late_head()
+                                }
                             };
 
                             receivers.push(BroadcastStream::new(receiver).map(|msg| {