Fixes sibling explosion bug caused by forwarding coordination
requests during handoff

Forward port of this issue from 1.4 to 2.0:
basho/riak_kv#1046

Original fix is:
https://github.com/basho/riak_kv/commit/d3c8c4b634446abf346e76f0b97c3cb784be35d1

There is a matching PR for riak_core
Merge pull request #1062 from basho/bug/gg/gh1046-forwardport

Fixes sibling explosion bug caused by forwarding coordination

Reviewed-by: seancribbs