Fix race condition in StreamTransferTask that could lead to
infinite loops and premature sstable deletion

patch by benedict; reviewed by yukim for CASSANDRA-7704
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/streaming/StreamTransferTask.java
	test/unit/org/apache/cassandra/streaming/StreamTransferTaskTest.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk