Switch from background jobs to using tasks

We want to finally move from Sidekiq background jobs for indexing or deletion to the Tasks approach for the following main reasons:

• To avoid blocking Sidekiq.
• Indexer will be able to control its own concurrency. Right now, Rails has no idea if the indexer can accept more indexing operations.
• We can communicate with the indexer omnidirectional instead of bidirectional, simplifying the overall architecture.