[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).

Sounds good, for this PR we could keep the NOTICE file to be inline with others. 
@brandtg, could you add copy of this module NOTICE file to the top level one like other modules?