coordinator: Fix test races caused by concurrent repository creation

Because most of our tests won't ever create repository records in the database when they set up a repository, we have a workaround in the coordinator which creates database records for repositories whenever we see a repository-scoped message. This approach is currently racy though: in case we have multiple concurrent repository-scoped RPC calls, we'll try to create the same repository multiple times. The result is that one of both RPCs will fail with an error.

Fix this issue by ignoring RepositoryExistsErrors when force-creating the database entries.

Fixes #3745 (closed)