Make CQLSSTableWriter sync within partitions

patch by slebresne; reviewed by blerer for CASSANDRA-7360
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/TreeMapBackedSortedColumns.java
	src/java/org/apache/cassandra/io/sstable/AbstractSSTableSimpleWriter.java
	src/java/org/apache/cassandra/io/sstable/CQLSSTableWriter.java
	src/java/org/apache/cassandra/io/sstable/SSTableSimpleUnsortedWriter.java