This implements the actual behavior the loader-core (dvcs ones) exposes. This
will allow to configure other loaders with the same retry (currently limited to
`content_add` here for discussion purposes [1]).
This should allow to:
- take care of issues like [2] (hash collision in package loaders otherwise
taken care of in dvcs loaders)
- clean up some more the current loader-core (original loader-core wraps the
storage's entrypoints with `send_{content,origin,...}` methods which adds
that retry policy)
[1] I will complete the implementation if we agree to continue in that
direction
[2] https://sentry.softwareheritage.org/share/issue/d4f1208b7eec4b43b11e38494ff039cc/