Test removal/restore from objstorage with swh-alter

We now test that swh alter remove actually removes content from the primary objstorage and from extra objstorages. We also test that swh alter recovery-bundle restore actually restores content to the objstorage.

To do this, we need to compute hashes (only SHA1 for now) of the content bytes. We extract these bytes from the recovery bundle for convenience, but this is a fairly slow operation. Therefore we only take a random subset (currently 5) of the removed Content objects in order to reduce the tests runtime.

In order to test extra objstorages, we need to also spin a content replayer. We adapt what was made for the mirror to do so.

We also switch to the new configuration format required by swh-alter v0.0.6.

Depends swh-alter!15 (merged)