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/block_verification.rs

@@ -1242,8 +1242,9 @@ fn load_parent<T: BeaconChainTypes>(
     let result = if let Some(snapshot) = chain
         .snapshot_cache
         .try_write_for(BLOCK_PROCESSING_CACHE_LOCK_TIMEOUT)
-        .and_then(|mut snapshot_cache| snapshot_cache.try_remove(block.parent_root()))
-    {
+        .and_then(|mut snapshot_cache| {
+            snapshot_cache.get_state_for_block_processing(block.parent_root())
+        }) {
         Ok((snapshot.into_pre_state(), block))
     } else {
         // Load the blocks parent block from the database, returning invalid if that block is not