This also adds a 'has_visits' boolean field to origin documents,
which will be set to True as soon as a visit for the origin is
seen.

'cursor' has a different meaning in our components using postgresql;
this renaming will avoid confusion when adding token-based pagination
to these components.

It keeps failing and I don't know what it is.

Related D2082

For some reason tests pass when run out of tox.

Like swh-storage, so multiple backends can be tested with the same
base test class.