Define messages for riak_dt clients.
Rename some DT messages for length and collision-prevention.
Allow SetOp and MapOp to have multiple arguments for ADD and REMOVE.

This makes the on-the-wire representation slightly smaller if the
client coalesces them.
Add codec module for data types and tweak some typespecs in the KV codec.
Export the riak_dt messages in Python.
Add dialyzer.
Allocate codes for the datatype messages.
Rename some messages for consistency with other proto files.
Add real update response message and codecs.