Add cleanup routines for underlying http transport

[andrewstucki]

This PR does a couple of things:

1. It limits the amount of connections we actually allow the undelrying http.Transport to initialize in the rpadmin client and sets some sane defaults based for idle connection timeouts based off of http.DefaultTransport
2. It introduces a Close method that flushes all idle connections in the connection pool. This allows us to manually close all pooled connections when a client is no longer needed.