Update CQLSSTableWriter to allow parallel writing of SSTables on the same table within the same JVM

Patch by Carl Yeksigian, reviewed by Benjamin Lerer for CASSANDRA-7463
Merge branch 'cassandra-2.1' into trunk
Update CQLSSTableWriter to allow parallel writing of SSTables on the same table within the same JVM

Patch by Carl Yeksigian, reviewed by Benjamin Lerer for CASSANDRA-7463