Reuse channels

One of the things i've noticed with a fairly large salt install is that the majority of the master's time seems to be authenticating requests. After some digging it is apparent that we end up creating and destroying zmq channels all over creation. This is an attempt to allow for session re-use. The objective is to re-use the same channel for all requests in a single execution/thread/process-- as having a single long running connection back to the master is a much larger job.