[feature request] pool connection delegate fn

[seanjensengrey]

When I create a pool, I'd like to be able to pass in a function that is periodically called that returns a set of hosts for the pool to use.

The use case is, that I would implement a function that talks to the riak-mesos endpoints url to find the current set of nodes that the cluster is running. This would obviate the need for talking through a proxy for long running services, w/o special casing how that node list is derived.