[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).

Ah, I think we have things somewhat backwards. Right now all of our submodules have LICENSE and NOTICE files, but I guess these only need to be at the top level? Helix has a single source release and per-module binary releases.