Update versions and NEWS file for 2.1.0 release
Improve stress workload realism

patch by benedict; reviewed by tjake for CASSANDRA-7519
Merge branch 'cassandra-2.1.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	build.xml
	debian/changelog
	tools/stress/src/org/apache/cassandra/stress/settings/SettingsCommandPreDefinedMixed.java
	tools/stress/src/org/apache/cassandra/stress/settings/SettingsCommandUser.java