HELIX-141: Autorebalance does not work reliably and fails when replica>1
add missing apache header
HELIX-148: Current preferred placement for auto rebalace is suboptimal for n > p
HELIX-150: Auto rebalance might not evenly distribute states across nodes
HELIX-155: PropertyKey.instances() is wrongly wired to CONFIG type instead of INSTANCES type
HELIX-134: refactor helix manager to handle zk session expiry more reliably. add more tests for new cluster manager impl
Added helix-api project
Added NodeInstance API
Merge branch 'helix_api_refactoring' of https://git-wip-us.apache.org/repos/asf/incubator-helix into helix_api_refactoring