Cleanup and optimize collation and slice iterators

patch by Aleksey Yeschenko and Benedict Elliott Smith; reviewed by
Aleksey Yeschenko for CASSANDRA-7107
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/ArrayBackedSortedColumns.java
	src/java/org/apache/cassandra/db/AtomicBTreeColumns.java
	src/java/org/apache/cassandra/db/filter/NamesQueryFilter.java