Fixes sibling explosion bug caused by forwarding coordination requests during
handoff

Forward port of this issue from 1.4 to 2.0:
https://github.com/basho/riak_kv/issues/1046

Original fix is:
https://github.com/basho/riak_core/commit/76cbdc24daa74e7976c6e15d48c1bfedde294183

There is a matching PR for riak_kv
Merge pull request #672 from basho/bug/gg/gh1046-forwardport

Fixes sibling explosion bug caused by forwarding coordination requests d...

Reviewed-by: seancribbs