mysqli: Retry on deadlock-found error

If MySQL error #1213 (deadlock found when trying to get lock) is returned
from query, it is safe to retry the query.

This patch will retry queries up to two times for a total of three. If the
query cannot be executed without error #1213 after the third time, the error
is logged as usual, and the system continues with the failed query.
Merge pull request #493 from greezybacon/feature/retry-queries

mysqli: Retry on deadlock-found error

Reviewed-By: Peter Rotich <peter@osticket.com>