Verify Index Exists Locally Before Handoff

Altered the should_handoff check (called by the new [handoff hook](basho/riak_kv#711)) to return false until all indexes expected to exist on a node handoff exist (respond to a solr ping). The expected indexes are gathered by pulling the previous nodes from pending changes, gets all indexes from each remote node (by disk), and verifying those index exist as solr cores.

This resolves issue #233, where previous, yokozuna would busy-wait when a put was indexed by a handoff.