Safer Index Updates

[brantburnett]

Some cases when updating indexes are still unsafe because they require dropping and recreating the index without an available replica. It would be nice to be able to create the new index first before dropping the old one in this case, though that does create naming conflicts which must be managed.

[brantburnett]

It may be best to simply make this a Couchbase Server 6.5 only feature via #56