database/sql: Retry with a fresh connection after maxBa...

database/sql: Retry with a fresh connection after maxBadConnRetries

Previously if the connection pool was larger than maxBadConnRetries and
there were a lot of bad connections in the pool (for example if the
database server was restarted), a query might have failed with an
ErrBadConn unnecessarily.  Instead of trying to guess how many times to
retry, try maxBadConnRetries times and then force a fresh connection to be
used for the last attempt.

At the same time, lower maxBadConnRetries to a smaller value of 4 now that
it's not that important to retry so many times from the free connection
list.