make prioritized executor service listenable
allow forcing all tasks to be prioritized
make MetricsEmittingExecutorService listenable
use Guava's ForwardingExecutorService instead of DelegatingExecutorService
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
Merge pull request #531 from metamx/listenable-prioritized-executorservice

make prioritized executor service listenable
[maven-release-plugin] prepare release druid-0.6.110
[maven-release-plugin] prepare for next development iteration