Don't panic in forkchoiceUpdated handler (#3165)

## Issue Addressed

Fix a panic due to misuse of the Tokio executor when processing a forkchoiceUpdated response. We were previously calling `process_invalid_execution_payload` from the async function `update_execution_engine_forkchoice_async`, which resulted in a panic because `process_invalid_execution_payload` contains a call to fork choice, which ultimately calls `block_on`.

An example backtrace can be found here: https://gist.github.com/michaelsproul/ac5da03e203d6ffac672423eaf52fb20

## Proposed Changes

Wrap the call to `process_invalid_execution_payload` in a `spawn_blocking` so that `block_on` is no longer called from an async context.

## Additional Info

- I've been thinking about how to catch bugs like this with static analysis (a new Clippy lint).
- The payload validation tests have been re-worked to support distinct responses from the mock EE for newPayload and forkchoiceUpdated. Three new tests have been added covering the `Invalid`, `InvalidBlockHash` and `InvalidTerminalBlock` cases.
- I think we need a bunch more tests of different legal and illegal variations

beacon_node/beacon_chain/src/state_advance_timer.rs

@@ -172,7 +172,7 @@ async fn state_advance_timer<T: BeaconChainTypes>(
 ///
 /// See the module-level documentation for rationale.
 fn advance_head<T: BeaconChainTypes>(
-    beacon_chain: &BeaconChain<T>,
+    beacon_chain: &Arc<BeaconChain<T>>,
     log: &Logger,
 ) -> Result<(), Error> {
     let current_slot = beacon_chain.slot()?;