Update for multiple snapshot OSTRICH

[opelgrin]

This PR includes the done during my PhD to support multiple snapshots OSTRICH.

[rubensworks]

Thanks @opelgrin, this looks great!

Before I merge, just some small questions:

1. Did anything change to the JavaScript API that warrant a change to the README? If yes, I'm happy to do this myself, as I know you may not be able to make time for this. But some pointers may help.
2. It looks like the gitmodules points to your fork of ostrich on branch delta-timestamps, but it appears this branch does not exist on github. Is it one that still needs to be pushed? Or is https://github.com/rdfostrich/ostrich/ on the master branch already up-to-date, and could that one be used instead?

[opelgrin]

Concerning point 1, I believe it works with Node 18 but not newer versions (and probably older versions as well, but I didn't test it). The Javascript API tend to be unstable for native addons it seems.
As for point 2, OSTRICH's master branch is up to date and we should use it.

[rubensworks]

Ok great, thanks @opelgrin!