[libasync] An asynchronous dispatch library.

The library defines an interface for asynchronously waiting on signals,
posting delayed tasks, or queuing packets on a port.

The library includes a thread-safe message loop based implementation of
this interface which can be used out of box. Clients can also write
their own dispatchers as long as they conform to the interface defined
by |async_ops_t|. Moreover, multiple dispatchers may coexist within
the same process.

This abstraction is intended to decouple clients which require asynchronous
dispatch from the specific implementation details of any particular
dispatcher.

Change-Id: I8cb107437d29ce5acd7093dee6ed1c6ce3779392