packets: reset read deadline before conn check (#964)
packets: reset read deadline before conn check (#964)

* packets: reset read deadline before conn check

If a MySQL connection has been configured with a short `ReadTimeout`,
each read from the TCP connection will be preceded by a
`SetReadDeadline` call, which lingers until the next `SetReadDeadline`.

This can be an issue if the connection becomes stale after staying too
long in the connection pool, because when we attempt to perform a stale
connection check, the Go runtime scheduler will return a timedout error
from the scheduler itself, without letting us get to the kernel to
perform the non-blocking read.

To fix this, reset the read deadline before we perform the connection
check.

* packets: set a 0 deadline

Go-SQL-Driver/MySQL,methane/mysql,methane/mysql,go-sql-driver/mysql,VividCortex/mysql,VividCortex/mysql