Batch up master tasks to create, mount, and delete snapshots #81846
Description
Moving an index into the cold or frozen tier involves creating a dedicated snapshot and then mounting that snapshot which is then ultimately deleted. The snapshot itself is usually trivial, but creating a snapshot involves some master tasks that are not batched, as does restoring/mounting/deleting a snapshot. If a large number of indices move into the cold or frozen tiers at the same time then it might take significant time to work through all these tasks. For instance, these tasks can bunch up when rollovers are scheduled at particular times rather than being size-based, or if the policy definition changes triggering a large number of phase transitions at once, or if the cluster recovers after a long period of unavailability.
We should try and process these master tasks in batches.
Relates #77466