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/errors.rs

@@ -26,6 +26,7 @@ use state_processing::{
 };
 use std::time::Duration;
 use task_executor::ShutdownReason;
+use tokio::task::JoinError;
 use types::*;
 
 macro_rules! easy_from_to {
@@ -170,6 +171,8 @@ pub enum BeaconChainError {
     CannotAttestToFinalizedBlock {
         beacon_block_root: Hash256,
     },
+    RuntimeShutdown,
+    ProcessInvalidExecutionPayload(JoinError),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);