[HELIX-279] Apply gc handling fixes to main ZKHelixManager class
Merge branch 'helix-0.6.2-release' of https://git-wip-us.apache.org/repos/asf/incubator-helix into helix-0.6.2-release

Conflicts:
	helix-admin-webapp/src/test/java/org/apache/helix/tools/TestHelixAdminScenariosRest.java
	helix-core/src/test/java/org/apache/helix/integration/TestCustomizedIdealStateRebalancer.java
[HELIX-279] Apply gc handling fixes to main ZKHelixManager class, adding back HelixManager#ALLOW_PARTICIPANT_AUTO_JOIN and mark it deprecated
[HELIX-273] Rebalancer interface should remain unchanged in 0.6.2, add test assert, rb=14915
Merge branch 'helix-0.6.2-release' of https://git-wip-us.apache.org/repos/asf/incubator-helix into helix-0.6.2-release