Ensure threads aren't forked in withTransaction

[brandonchinn178]

SqlBackend is not thread safe, so one should not use the same SqlBackend in multiple threads (ref). This means that the following program will have problems:

withTransaction $ do
  -- forkIO from UnliftIO.Concurrent
  forkIO $ insert $ ...
  insert $ ...

Make sure this can't happen

[brandonchinn178]

I'm not sure how we'd be able to enforce this at the type level, so maybe we should just store myThreadId in the environment and check in runQueryRep that the current thread ID is the same? That way, we can deterministically fail if someone tries to fork within withTransaction rather than having a race condition.