Add support for rotating connection details

[tomalexander]

It would be nice if the Pool supported rotating connection details. Our use-case would be using the hashicorp vault database secrets engine to generate postgres credentials on-the-fly. These credentials inherently have a limited life-span so when they expire we'd need to rotate them. As a work-around we were going to swap out the pool with a new pool but #290 prevents that.

One possible way to implement it, would be:

1. expose a function to set the connection details
2. when that function is called, all new calls to acquire() would return connections with the new connection details set
3. as connections with the old details are returned to the pool they get discarded

I could see this also being useful for fail-overs but thats not our use-case so idk

[elprans]

Possibly the simplest solution would be to factor out connection spec into a Pool method and allow subclassing it. Then we could add a recycle() method to the pool to indicate that all open connections should expire upon release() to trigger graceful failover.