test and efficiently handle multiple source sequence retrievals

The new tree based data source browser dialog in JAL-1075 allows multiple sources to be selected, and the underlying machinery allows cross-source querying. Tests are need to improve the efficiency of querying (e.g. not querying sources with IDs that are known a priori to not be from that authority), and to ensure that results from cross-authority queries and cross-source queries are properly annotated with the source and the authority/accession that yielded the data.