that must not send a response

Problem:- When using wsrep (w/ galera) and issuing commands that can
cause deadlocks, deadlock exception errors are sent in responses to
commands such as close prepared statement and close connection which,
by spec, must not send a response.

Solution:- In dispatch_command, we will handle COM_QUIT and COM_STMT_CLOSE
commands even in case of error.

Patch Credit:- Jaka Močnik