Allow updating individual subfields of UDT

Since user defined types were implemented in CASSANDRA-5590 as blobs (you have to rewrite the entire type in order to make any modifications), they can't be safely used without LWT for any operation that wants to modify a subset of the UDT's fields by any client process that is not authoritative for the entire blob.

When trying to use UDTs to model complex records (particularly with nesting), this is not an exceptional circumstance, this is the totally expected normal situation.

The use of UDTs for anything non-trivial is harmful to either performance or consistency or both.

edit: to clarify, i believe that most potential uses of UDTs should be considered anti-patterns until/unless we have field-level r/w access to individual elements of the UDT, with individual timestamps and standard LWW semantics