AsyncHttpClient/async-http-client — pull request #624 by slandelle (2014-07-16 12:32:12+00:00)
Title: Refactor Netty channel pool for 1.9, close #201, close #623

Introduce ChannelManager, in charge of logic and limits.

Limits are handled with Semaphores:
* `tryAcquire` is preemptively done when we're about to open a new Channel
* `release` is either done if connection failed, or when the Handler is notified that the Channel is closed

We now properly support maxConnectionsPerHost.