Include high level sstables in lower level compactions if not compacted for a while.

Patch by marcuse, reviewed by kohlisankalp and yukim for CASSANDRA-7414
Merge branch 'cassandra-2.0' into cassandra-2.1

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