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/http_api/src/metrics.rs

@@ -29,4 +29,8 @@ lazy_static::lazy_static! {
         "http_api_beacon_proposer_cache_misses_total",
         "Count of times the proposer cache has been missed",
     );
+    pub static ref HTTP_API_BLOCK_BROADCAST_DELAY_TIMES: Result<Histogram> = try_create_histogram(
+        "http_api_block_broadcast_delay_times",
+        "Time between start of the slot and when the block was broadcast"
+    );
 }