Improve schema merge performance

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-7444
Merge branch 'cassandra-2.0' into cassandra-2.1
Fix NPE when unknown prepared statement ID is used

Patch by Tyler Hobbs; reviewed by Sylvain Lebresne for CASSANDRA-7454
Merge branch 'cassandra-2.1.0' into cassandra-2.1
merge from 2.1.0
Adjust MT depth based on # of partitions validating

patch by yukim; reviewed by jbellis for CASSANDRA-5263
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/compaction/LeveledManifest.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Optimise CellName/Composite comparisons for NativeCell

patch by Jake Luciani and Benedict Elliott Smith for CASSANDRA-6755
Merge CHANGES.txt from 2.1.0
fix CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/compaction/LeveledManifest.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1
rename
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1