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/errors.rs

@@ -137,6 +137,10 @@ pub enum BlockProductionError {
     /// The `BeaconChain` was explicitly configured _without_ a connection to eth1, therefore it
     /// cannot produce blocks.
     NoEth1ChainConnection,
+    StateSlotTooHigh {
+        produce_at_slot: Slot,
+        state_slot: Slot,
+    },
 }
 
 easy_from_to!(BlockProcessingError, BlockProductionError);