close race condition with watch and readdir

[krisselden]

Calling close on a watcher closes currently watched directories but does not account for asynchronous callbacks.

Easiest way to reproduce is to create a watcher and try to close it right away.

[krisselden]

To clarify, if you close during an async op who's callback calls Watcher.prototype.watchdir then Watcher.prototype.watchdir should guard against the state of being closed.