making this work for multiple queries and search results from rocksdb indexes

[bluegenes]

Differences to account for:

1. no query_filename in output gather file
2. no rank in output gather (though we will get this with core updates coming in MRG: use core Collection and Select for sig loading sourmash-bio/sourmash_plugin_branchwater#197)
3. multiple queries = need to load each query as it is needed

[ctb]

in multigather output:

Differences to account for:

1. no `query_filename` in output gather file

what!? argh.

2. no `rank` in output gather  (though we will get this with core updates coming in [WIP: use core Collection and Select for sig loading sourmash-bio/sourmash_plugin_branchwater#197](https://github.com/sourmash-bio/sourmash_plugin_branchwater/pull/197))

the order should be the rank, tho, right?

3. multiple queries  = need to load each query as it is needed

well, yes.