[1] Without this diff, mercurial loader run within docker fails with multiple
errors (in order, one error appears after another):
- TypeError: can not serialize 'map' object
- TypeError: can not serialize 'set' object
So this diff fixes those:
- `map` is not ok when calling `storage.content_missing`
- set are not ok when calling `storage.{revision|release}_missing`
No idea why the tests do not capture any of those issues though.
I'm just unstucking this so people can run it within docker.
[1] The initial problem was along those lines:
```
- swh.core.api.RemoteException: <RemoteException 500 AttributeError: ["'dict' object has no attribute 'url'"]>
```
where the self.origin being written to storage was a dict instead of an Origin
model object [1].
That error is now gone with the current loader-core at least v0.2.0.