Add #riak_core_fold_req_v2 record, part 1 of 2

NOTE: This PR is incomplete. I've created it for ease of discussion this week in Cambridge.

Move definition of ?FOLD_REQ to v2.

Add functions to riak_core_util.erl to create v2 versions of the
record: arity 1 (upgrade #riak_core_fold_req_v1 record to the v2
record), arity 2 (args = the two record elements of the v1 record)
and arity 4 (args = the four record elements of the v2 record).

Disable request forwarding in riak_core_vnode:forward_or_vnode_command()
if the request explicitly contains not-forwardable annotation.