Update system.peers correctly when relocating tokens.

Patch by brandonwilliams, reviewed by tjake for CASSANDRA-7126
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1' into trunk