Fix regression in SSTableRewriter causing some rows to become unreadable during compaction

patch by marcus and benedict for CASSANDRA-8429
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
	src/java/org/apache/cassandra/io/compress/CompressionMetadata.java
	src/java/org/apache/cassandra/io/sstable/format/big/BigTableWriter.java