Metrics for prepared stmt usage and eviction

Patch by Robbie Strickland; review by Tyler Hobbs for CASSANDRA-7930
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/QueryProcessor.java
	src/java/org/apache/cassandra/metrics/CqlStatementMetrics.java