Merge groupable mutations in TriggerExecutor#execute()

patch by Aleksey Yeschenko and Sergio Bossa for CASSANDRA-7047
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/service/StorageProxy.java
	src/java/org/apache/cassandra/triggers/TriggerExecutor.java
	test/unit/org/apache/cassandra/triggers/TriggersTest.java