[ML] Allow stop unassigned datafeed and relax unset upgrade mode wait

[droberts195]

These two changes are interlinked.

Before this change unsetting ML upgrade mode would wait for all
datafeeds to be assigned and not waiting for their corresponding
jobs to initialise. However, this could be inappropriate, if
there was a reason other that upgrade mode why one job was unable
to be assigned or slow to start up. Unsetting of upgrade mode
would hang in this case.

This change relaxes the condition for considering upgrade mode to
be unset to simply that an assignment attempt has been made for
each ML persistent task that did not fail because upgrade mode
was enabled. Thus after unsetting upgrade mode there is no
guarantee that every ML persistent task is assigned, just that
each is not unassigned due to upgrade mode.

In order to make setting upgrade mode work immediately after
unsetting upgrade mode it was then also necessary to make it
possible to stop a datafeed that was not assigned. There was
no particularly good reason why this was not allowed in the past.
It is trivial to stop an unassigned datafeed because it just
involves removing the persistent task.

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

Although I labelled this >enhancement it's actually critical to users of the migration assistant in 6.7 getting a good experience reindexing ML indices in non-trivial production environments. Without this change unsetting upgrade mode after reindexing an ML index could hang, leading to poor user experience for the current reindex operation and also problems starting the reindexing for the next ML index too.