[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).

So is this going to separate source zip? If this would be under the parent umbrella of main source the NOTICE need to be on top level directory [1]

[1] http://www.apache.org/legal/src-headers.html