skipped_content_missing is not implemented by the in-memory storage

The in-memory storage, implemented in swh/storage/in_memory.py is a clone of the pgsql storage (swh/storage/storage.py and swh/storage/db.py).

However, it's missing some features, like the skipped_content_missing, which lists content entries that we are missing the actual data.

This is a rarely used method, but it's useful for tests. Right now, the test test_skipped_content_add in swh/storage/tests/test_in_memory.py is disabled because of this.

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Migrated from T1633 (view on Phabricator)