Support nul/non-existant fields in UDT

patch by slebresne; reviewed by iamaleskey for CASSANDRA-7206
Correctly name FileCache metrics.

Patch by Nick Bailey, reviewed by brandonwilliams for CASSANDRA-7294
Merge branch 'cassandra-2.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk