Improve compaction of repaired/unrepaired sstables

Patch by marcuse; reviewed by Aleksey Yeschenko and Sankalp Kohli for CASSANDRA-8004
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
	src/java/org/apache/cassandra/db/compaction/LeveledCompactionStrategy.java
	src/java/org/apache/cassandra/db/compaction/LeveledManifest.java
	src/java/org/apache/cassandra/db/compaction/SizeTieredCompactionStrategy.java
	test/unit/org/apache/cassandra/db/compaction/CompactionsTest.java
	test/unit/org/apache/cassandra/db/compaction/LeveledCompactionStrategyTest.java