Use advanced state for block production (#2241)

## Issue Addressed

NA

## Proposed Changes

- Use the pre-states from #2174 during block production.
    - Running this on Pyrmont shows block production times dropping from ~550ms to ~150ms.
- Create `crit` and `warn` logs when a block is published to the API later than we expect.
    - On mainnet we are issuing a warn if the block is published more than 1s later than the slot start and a crit for more than 3s.
- Rename some methods on the `SnapshotCache` for clarity.
- Add the ability to pass the state root to `BeaconChain::produce_block_on_state` to avoid computing a state root. This is a very common LH optimization.
- Add a metric that tracks how late we broadcast blocks received from the HTTP API. This is *technically* a duplicate of a `ValidatorMonitor` log, but I wanted to have it for the case where we aren't monitoring validators too.

beacon_node/beacon_chain/src/test_utils.rs

@@ -408,7 +408,7 @@ where
 
         let (block, state) = self
             .chain
-            .produce_block_on_state(state, slot, randao_reveal, Some(graffiti))
+            .produce_block_on_state(state, None, slot, randao_reveal, Some(graffiti))
             .unwrap();
 
         let signed_block = block.sign(