This means that, when not saturated, objects in the pool will be reused
more often.
This behavior allows you to create a pool of objects which in turn have
some kind of time outs themselves. For instance, objects which keep an
underlying TCP connection open until they're idle for some time. An
example of this is `Net::HTTP::Persistent`.
See also issue #20.
If you did nested checkouts the inner use would check the connection
back to the pool and then the outer checkout could be using a connection
at the same time with something else. See issue #14.