Fix spin loop in AtomicSortedColumns

patch by graham sanderson and benedict; reviewed by yukim for
CASSANDRA-7546
Merge branch 'cassandra-2.1' into trunk
Fix handling of EXECUTE with skip_metadata=false

patch by Aleksey Yeschenko and Jake Luciani; reviewed by Sylvain
Lebresne
Merge branch 'cassandra-2.1' into trunk
Merge remote-tracking branch 'origin/trunk' into 7563v3-udf-types
minor refactoring