Perform handoff check only when Yokozuna is enabled

The handoff check only needs to be performed when Yokozuna is enabled.
Checking it in all cases doesn't break anything at the moment but if the
code was changed in the future it could cause KV handoff breakage when
Yokozuna is disabled. In that case, don't put KV in harms way. Don't
perform the check.
Don't make metadata call when service is down

Check for metadata convergence only when the Yokozuna service is running
on the other side.

If the service is not running on the other side then it indicates mixed
cluster or that Solr has not finished starting on the remote node. In
both cases there is no need to block handoff on metadata convergence
because there is no way to index the data anyways. That is, the metadata
check is currently only there to make sure that the `search_index`
properties have been propagated so that indexing events are not missed.
If Yokozuna doesn't exist or Solr is not up then no need to worry about
bucket properties since the indexing will fail anyways.
Merge pull request #275 from basho/bugfix/rz/yz-should-handoff

Fix handoff check in mixed cluster scenario