Extend triggers to support CAS updates

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-6882
Merge branch 'cassandra-2.0' into cassandra-2.1

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