Re-introduce the 1.4 counters API

Re-add the 1.4 API.

Support legacy counters at the FSM, Vnode, kv_crdt level.

I really think you don't need to review `riak_kv_gcounter` and `riak_kv_pncounter` as they are practically unchanged from 1.4 (The 2.0 API requires `update` returns {ok, Val} is the only change.)

The upgrade support is untested as upgrade seems to be broken. But running legacy clients against 2.0 has been tested (just run the riak_test for counters.)