Calls to deref. are blocking when agent is updating its value

see https://github.com/jdantonio/concurrent-ruby/blob/master/lib/concurrent/agent.rb#L163

I'll submit PR later. What is the schedule for 0.6 release? how much time do I have If I would like to get this fix in?

- [ ] do not block reading when updating
- [ ] does not allow to execute two updates at the same time

Other:
- [ ] creates another thread for each post call see https://github.com/jdantonio/concurrent-ruby/blob/master/lib/concurrent/agent.rb#L164
- [ ] do not send subsequent posts on failure, clojure agent doc: _"If any exceptions are thrown by an action function, no nested dispatches will occur, and the exception will be cached in the Agent itself."_
- [ ] support `send` and `send-off`for blocking IO functions