Simplify thread management with a bounded thread count

[edahlgren]

Simpler thread management:

• set n worker threads
• serve all requests to worker threads via a shared mvar queue
• handle a request on a worker thread with a callback through the zmq Router
• eliminate pipelining

Benefits:

• less gc-ing of threads due to a fixed thread count

Potentially costs:

• same requests will now all go to riak, all deserializing responses, potentially elevating gc

[wmoss]

This looks reasonable (except I don't understand the change I commented on).