Implement "Refresh Key List via Search Query" capability

[dmitrizagidulin]

For bucket types and buckets that are indexed with Riak Search, Solr provides an alternate mechanism to generate cached key and bucket lists.

(Note: Solr automatically keeps an rb field (riak bucket) for each document it indexes, which provides a convenient filter to get all the keys in a bucket via a query).

Implement another /refresh endpoint - /refresh_keys/source/search, that populates a cached list from a 'get all documents' Search query.