Add writable() to AsyncTransport

Add writable() to AsyncTransport

Summary:
This is useful for checking if it's possible to still write to a transport,
even if its read side is closed (for transports that support half shutdown,
like AsyncSocket). Default implementation just returns true for now (up
to implementers to override).

Reviewed By: yfeldblum

Differential Revision: D4982649

fbshipit-source-id: 0a9a2e2b745ea3db57e9f151f3a8634e1bda2465