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]

@jamwt can you comment on why you originally designed this function to return?

[edahlgren]

I'm curious as to why you think that change matters. @jamwt's version is semantically equivalent to void $ forkIO. Perhaps the first version of montage was trying to avoid "last statement in a 'do' block must be an expression" issues.

[wmoss]

Your new version doesn't forkIO though. It seems unnecessary to do it (since we're calling forkIO at the top level) but I was curious why it was done originally.

[wmoss]

I now understand (from IRC).

[wmoss]

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