Use live sstables in snapshot repair if possible

patch by Jimmy Mårdell; reviewed by yukim for CASSANDRA-8312
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java