Fix UPDATE updating PRIMARY KEY columns implicitely

patch by slebresne; reviewed by iamaleksey for CASSANDRA-6782
Fix IllegalArgumentException when upgrading from 1.2 with SC

patch by slebresne; reviewed by vijay2win for CASSANDRA-6733
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/UpdateStatement.java
	src/java/org/apache/cassandra/db/columniterator/IndexedSliceReader.java
Merge branch 'cassandra-2.1' of https://git-wip-us.apache.org/repos/asf/cassandra into cassandra-2.1

Conflicts:
	CHANGES.txt
	test/unit/org/apache/cassandra/io/compress/CompressedRandomAccessReaderTest.java
Update java-driver to 2.0 final for stress

patch by djatnieks; reviewed by slebresne for CASSANDRA-6777