Nifty fbcode inheritance refactor

This pull adds changes to nifty which make it possible for the variants of NettyServerTransport and NiftyDispatcher in the nifty-fbcode repo to inherit from the same classes in the core nifty server.

A couple other things in here:
- add a "stop future" to NettyServerTransport which can be used to listen for / wait on the server to stop (since NettyServerTransport.start() does not block)
- add a couple convenience overrides of start() to NettyServerTransport for common usage