Properly handle unrecognized opcodes and flags

Patch by Tyler Hobbs; reviewed by Sylvain Lebresne for CASSANDRA-7440
Merge branch 'cassandra-2.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk
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' into trunk