Verify Index Exists Locally Before Handoff

Altered the should_handoff check (called by the new [handoff hook](https://github.com/basho/riak_kv/pull/711)) to return false until all indexes expected to exist on a node handoff exist (respond to a solr ping).

This resolves issue #233, where previous, yokozuna would busy-wait when a put was indexed by a handoff.