multiple thread for topic op queuer

Conflicts:
	hedwig-server/src/main/java/org/apache/hedwig/server/common/ServerConfiguration.java
add leveldb as a persistence backend

Conflicts:
	hedwig-server/pom.xml
	hedwig-server/src/test/java/org/apache/hedwig/server/HedwigHubTestBase.java
	hedwig-server/src/test/java/org/apache/hedwig/server/delivery/TestThrottlingDelivery.java
some commits for hierarchical proposal
provide a snitch interface as a container for different implementations.
provide a helix based snitch, including helix based topic manager, persistence manager & subscription manager.