Expose off-heap memory usage stats

patch by Benjamin Lerer; reviewed by Aleksey Yeschenko for
CASSANDRA-7897
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/io/sstable/SSTableReader.java
	src/java/org/apache/cassandra/tools/NodeCmd.java
	src/java/org/apache/cassandra/utils/IFilter.java
	src/java/org/apache/cassandra/utils/obs/IBitSet.java