Translate bucket, or expect index name, in mapred_search

This PR changes mapred_search to lookup the index name associated with a bucket, instead of passing the bucket name to solr. The bucket name is also passed in a filter query, such that only keys from the bucket originally specified are returned from the index.

This PR also allows the specification of an index, instead of a bucket, which will produce all keys matching the query, regardless of bucket. To trigger this behavior, pass `{index, Index}` or `{struct, [{<<"index">>}, Index}]}` (mochijson2 format) instead of `Bucket` as the `Group` parameter to `mapred_search`.