Use project_id_from and project_id_to in the InitialIndexingEventWorker

Background

We are performing initial indexing for all projects of a namespace attached to a zoekt index. Since we can have now multiple zoekt_indices for a namespace. This approach could create redundant initial indexing tasks for a namespace having multiple zoekt_indices

Proposal

Use project_id_from and project_id_to values from the metadata column of zoekt_indices table. If there is no project_id_to field that means fetching all the projects from project_id_from. If there is no project_id_from field then fetch all projects of a namespace.