## 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
ae47a93c42