[ML] Fix possible race condition when starting datafeed

[droberts195]

The ID of the datafeed's associated job was being obtained
frequently by looking up the datafeed task in a map that
was being modified in other threads. This could lead to
NPEs if the datafeed stopped running at an unexpected time.

This change reduces the number of places where a datafeed's
associated job ID is looked up to avoid the possibility of
failures when the datafeed's task is removed from the map
of running tasks during multi-step operations in other
threads.

Fixes #51285

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[davidkyle]

I think #49467 may be caused by the same race

[dimitris-athanasiou]

LGTM

[droberts195]

I think #49467 may be caused by the same race

Maybe. If it was then there would have been an NPE logged on one of the external cluster nodes during the failed test. Unfortunately the Jenkins builds for the failures of #49467 have now gone, so we cannot download the external cluster logs to check.

[davidkyle]

I would have spotted in NPE in the logs when triaging #49467. That test timed out waiting for the datafeed to stop, my theory is the NPE took down the worker thread that is vital in stopping the DF so a stopped status was never reached.