fix race condition with merge and persist and sink adding

Conflicts:
	indexing-service/src/main/java/io/druid/indexing/common/task/IndexTask.java
	server/src/main/java/io/druid/segment/realtime/RealtimeManager.java
	server/src/main/java/io/druid/segment/realtime/plumber/RealtimePlumber.java
fix for code review comments
fix another code review comment
Merge pull request #549 from metamx/fix-sink

fix race condition with merge and persist and sink adding