[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).

Forgot to comment this one. Please move NOTICE file content to NOTICE in top level Helix directory. Then you can remove this one.

NOTICE file should be bundled into one when making releases and just copy this to main NOTICE will make packaging easier.