Task management

[clintongormley]

We need a task management API to allow management of long running tasks, like snapshot/restore, benchmarking, update-by-query etc.

The API should allow listing and aborting of ongoing tasks.

[1st]

+1. Waiting for this new feature!!!

[markharwood]

Part of killing long running tasks will be tied up in this new generic timeout handling mechanism: #4586

[shikhar]

Will this cover search requests? Since one can't know in advance very well if a search request will be long-running, I guess this task tracking would need to apply to all of them.

If this is not the plan, I think #7157 should be reopened.

[martijnvg]

@shikhar The task infrastructure will be generic enough that it can be adopted in the search api as well.

[efuquen]

👍 Not having this has been a pain.