Implement delete() in WineryObjStorage

We now allow to delete objects from Winery. The object is first marked as deleted in the shared base. This makes it inaccessible from further get() calls. If the object still in a RWShard, the relevant line in the objects table is deleted as well.

To take care of removing the data in ROShards, a dedicated host able to map images in read-write mode can then call:

swh objstorage winery clean-deleted-objects

This will zero out any deleted object present in the images and remove their key from the indices. Object keys will then be removed from the shared database.

Depends on !169 (merged)

Addresses swh-alter#4 (closed)