database/sql: check conn expiry when returning to pool, not when handing it out
With the original connection reuse strategy, it was possible that
when a new connection was requested, the pool would wait for an
an existing connection to return for re-use in a full connection
pool, and then it would check if the returned connection was expired.
If the returned connection expired while awaiting re-use, it would
return an error to the location requestiong the new connection.
The existing call sites requesting a new connection was often the last
attempt at returning a connection for a query. This would then
result in a failed query.
This change ensures that we perform the expiry check right
before a connection is inserted back in to the connection pool
for while requesting a new connection. If requesting a new connection
it will no longer fail due to the connection expiring.