Redis refusing to run MULTI or EXEC during script timeout may cause partial
transactions to run.
1) if the client sends MULTI+commands+EXEC in pipeline without waiting for
response, but these arrive to the shards partially while there's a busy script,
and partially after it eventually finishes: we'll end up running only part of
the transaction (since multi was ignored, and exec would fail).
2) similar to the above if EXEC arrives during busy script, it'll be ignored and
the client state remains in a transaction.
the 3rd test which i added for a case where MULTI and EXEC are ok, and
only the body arrives during busy script was already handled correctly
since processCommand calls flagTransaction
