Optimize single partition batch statements

patch by slebresne; reviewed by benedict for CASSANDRA-6737
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/BatchStatement.java
	src/java/org/apache/cassandra/cql3/statements/DeleteStatement.java
	src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java
	src/java/org/apache/cassandra/cql3/statements/UpdateStatement.java
	src/java/org/apache/cassandra/db/Mutation.java