Use ThreadLocalRandom and remove FBUtilities.threadLocalRandom

patch by benedict; reviewed by tjake for CASSANDRA-7934
ninja-fix CHANGES.txt
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/io/compress/CompressedRandomAccessReader.java
	src/java/org/apache/cassandra/service/QueryState.java