InitialIndexingEventWorker should process one batch at a time

Currently, InitialIndexingEventWorker processes all projects at the same time. If it happens so that we have a lot of repositories (similar to 2024-12-19: The PGBouncer Single Core per Node ... (gitlab-com/gl-infra/production#19033 - closed)), it might take extremely long time to finish.

Instead, we should only create N repositories during the run, and mark the index as initializing when we no longer have projects left.