forward COM_PING to MySQL

[altmannmarcelo]

Description

COM_PING is used to check if a connection is still open and reset it's wait/interactive_timeout.

Currently we are able to read COM_PING and return an OK_PACKET back, but we should also send a COM_PING in the upstream connection attached to the connection that received COM_PING in order to refresh the timeout on the upstream connection.

Change in user-visible behavior

Requires documentation change