Add column update delta histogram

Patch by Sankalp Kohli, review by jmckenzie for CASSANDRA-7979
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/AtomicSortedColumns.java
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/db/Memtable.java
	src/java/org/apache/cassandra/metrics/ColumnFamilyMetrics.java
Add column update delta histogram (2.1)

Patch by Sankalp Kohli, review by jmckenzie for CASSANDRA-7979
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/AtomicBTreeColumns.java
	src/java/org/apache/cassandra/db/Memtable.java