Add an in-memory scheduler for testing purpose

The code is not very pretty (having unhashable (albeit frozen) model objects is a bit sad), but it looks good enough for tests (can be used to replace postgres backend scheduler in swh-web tests for example).

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Looks good to me, thanks for implementing this !

I plugged the memory backend to swh-web tests but also in swh-lister CLI and everything works fine !

I think you can also remove the temporary postgresql backend in another commit.

approved this merge request

merged