Add a thread-local RequestContext

Currently just contains the remote address of the client connection that made the request
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Remove NiftyContext class

This file, added in a recent commit, actually became RequestContext, but I forgot to delete the original and committed it. It isn't used anywhere.
Make frame codec a configurable part of the server
Add custom nifty processors
Remove header transport option
Duplex protocol and transport factories
Integrate duplex protocol factories
Add convenience overrides for start() method on NettyServerTransport
Fix bug in the new frame decoder
Add 'final' keyword to ThriftMessage fields