noticeable pause when sourmash tries to load a ZipFile collection as an SBT

[ctb]

There's a noticeable pause while _load_sbt in _load_database tries to load a .zip file as an SBT (before failing and going on to load it as a .zip file, correctly). Not sure what's going on here.

Punted here as a remaining issue from #1483

[ctb]

maybe related to big memory peak on SBT loading #1197

[ctb]

looks like this was fixed by #1909, which oxidized ZipStorage!