Fix totalDiskSpaceUsed calculation

patch by yukim; reviewed by krummas for CASSANDRA-8205
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/io/sstable/SSTableDeletingTask.java
Merge branch 'cassandra-2.1' into trunk