Task hold/release needs to be flow-specific

[hjoliver]

Follow-up to #4238

[MetRonnie]

From that PR:

I think we will want to add the "flow label" into the tasks_to_hold set in order to handle the situation where we have the same task from different flows. E.G. the user might want to hold one flow but the another

[hjoliver]

Probably rather simple to implement. I'll take a look ...

[ColemanTom]

Just to check. Am I correct that being able to hold a specific flow, should also mean that cylc release shoul be able to operate on a specific flow too?

[hjoliver]

Yes, correct (I'll update the title).

[TomekTrzeciak]

What is the expected/planned behaviour when anther flow merges with the held one?

[hjoliver]

The merged task will be held, I think. Similarly, if you hold flow 3 (say) a merged task belonging to flows (2,3) should be held.