Allow idle connection count to exceed maxIdle for configurable time period

It would be nice to have a timeout that specifies how long a connection, on which close() has been called, should be held in the pool, even if this connection exceeds the maxIdle limit. This would make it possible to say something like "I know this is one more idle connection than I configured but keep it around for just a few more seconds in case we have a second peak so we don´t need to construct the connection again".

If this has a chance of getting into the pool, I would try to provide a patch that implements this so that, in best case, you only would have to do a code review.