Backport MultiSliceRequest

Patch by Ed Capriolo; Reviewed by tjake for CASSANDRA-7027
Handle overlapping multislices in thrift and cql

Patch by pcmanus and tjake; reviewed by tjake for CASSANDRA-7279
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	interface/cassandra.thrift
	src/java/org/apache/cassandra/thrift/CassandraServer.java
	test/unit/org/apache/cassandra/thrift/MultiSliceTest.java