This fixes a race whereby:
- User hits an endpoint.
- No cached transaction so executes main code.
- User hits same endpoint.
- No cache transaction so executes main code.
- Main code finishes executing and caches response and returns.
- Main code finishes executing and caches response and returns.
This race is common in the wild when Synapse is struggling under load.
This commit fixes the race by:
- User hits an endpoint.
- Caches the promise to execute the main code and executes main code.
- User hits same endpoint.
- Yields on the same promise as the first request.
- Main code finishes executing and returns, unblocking both requests.
