Use the connections in the pool uniformly

Use the connections in the pool uniformly

I feel this is a better use than using some more than other (e.g. in
term of bloat of the connections associated with the resources) and
gives a more predictable performance of the connection (there won't be
some cold, some hot).

Now there aren't really "unused connections" to single out in order to
shrink the pool. So keep a tally of the number of connections unused and
use a worker thread to close some if there are above minconn unused in a
period.