[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).

Since we release Helix as per-module binary bundles, personally I think having specific NOTICE files makes more sense -- that way, people can pick and choose which modules to use without needing to worry about dependencies present in other modules.