MINOR: checks: mysql-check: Add support for v4.1+ authentication

MySQL will in stop supporting pre-4.1 authentication packets in the future
and is already giving us a hard time regarding non-silencable warnings
which are logged on each health check. Warnings look like the following:

"[Warning] Client failed to provide its character set. 'latin1' will be used
as client character set."

This patch adds basic support for post-4.1 authentication by sending the proper
authentication packet with the character set, along with the QUIT command.