Use Async::Callback in Api::Server

Related to issue #7.
Clean up Async.hpp
Implement LocalAsyncHost and LocalTaskQueue

LocalAsyncHost is a new abstraction over epoll.
LocalTaskQueue replaces the nextTick() function.
Both classes are thread-local. The LocalTaskQueue class allows
thread-safe submission of tasks to a thread.

Related to issue #8.
Improve epoll implementation
Issue #7: Use Async::Callback in WriteAhead
Rewrite Api::Server to queue outgoing packets
SockStream: Allow only one concurrent r/w call
Use Async::Callback in os interface

Closes #7.