Delete unfinished compaction sstables incrementally

patch by thobbs; reviewed by yukim for CASSANDRA-6086
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/service/CassandraDaemon.java
	test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java