[ML] Check for missing delete tasks and recreate during nightly maintenance

[droberts195]

When an ML job is deleted a task is created to delete all the things that need deleting. However, this is not a persistent task, only a node-specific task. If the node running that task is shut down before the deletion completes then the job is left in a partially deleted state.

One option to fix this would be to make the deletion task a persistent task. However, this is a huge amount of development work.

A simpler alternative could be to have the nightly maintenance action check for jobs in the deleting state that don't have a corresponding deletion task, and start a new deletion task for them.

[elasticmachine]

Pinging @elastic/ml-core

[benwtrent]

A simpler alternative could be to have the nightly maintenance action check for jobs in the deleting state that don't have a corresponding deletion task, and start a new deletion task for them.

I am all for that.

A persistent task seems like overkill.