We should change search fold to take a list of `Filters`.  Each filter should become its own `fq` param.  You can repeat `fq` param as many times as your want and Solr will do an implicit conjunction.  The reason to make them separate is that filter queries may be cached in solr.  And the cache key is determined by the query string.  By concatenating the user filter + bucket filter you are making unique keys and thus the bucket filter query cached result cannot be shared across query requests.  It also means more entries in the cache table and thus more on-heap memory in Solr land.

Although, I do wonder if we should not bother adding this implicit filter at all.  If the user decides, during migration, to migrate multiple Riak Search buckets to the same index then he or she should understand that a query on that index can return data under all buckets.  If they don't like this they can change their query code to pass an explicit bucket filter.