[HELIX-470] Netty-based IPC layer

First-pass at an IPC layer for Helix. This provides cluster-scope-addressable messaging (e.g. send to all SLAVE partitions of a resource).

@kanakb, sorry looks like Helix already put NOTICE file in each module that released separately.
I guess if we package this separately then NOTICE for this module should stay but I think the top level NOTICE file should have copy of the content.

Does each NOTICE file in the sub module does not have copy to top NOTICE file?