Batch the update and delete operations in Task Manager

[mikecote]

From @kobelb

This would increase the performance by drastically reducing the likelihood of tasks themselves filling up the Elasticsearch write queue, and lets us execute more tasks concurrently by increasing the concurrent tasks per instance.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[gmmorris]

It seems the SavedObjectsRepository doesn't yet have support for balk delete, so I'm going to split this into three incremental PRs:

1. First PR will bulk up the update operation, as we already have support for bulkUpdate and that will also cover most of the work as adding support to these bulk operations in TM is the same which ever operation we choose to focus on first.
2. The second PR will add bulkDelete to the SavedObjectsRepository, which will require coordination with @elastic/kibana-platform
3. Finally, we'll use the new bulkDelete api to batch up the remove operation.

It is worth reprioritising parts 2 & 3 in this work, as we do not remove tasks anywhere near as often as we update them (Alerting, for example, reschedules all it's alerts so those tasks are only removed when an alert is disabled or deleted).
I'll bring this up in the sync