Enable downloading from multiple tdb databases
Enable downloading from multiple tdb databases

Previously we were using download_all to call vdb download separately on tdb, tdb_cdc, etc... and then merge results. This commit allows --database tdb tdb_cdc to merge these in tdb download. This should be significantly cleaner.
blab/nextstrain-db,blab/nextstrain-db,nextstrain/fauna,nextstrain/fauna