Evaluate concurrent and hourly limits once per branch

[rarkins]

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Today Renovate uses this logic:

• Calculate concurrent and hourly limits remaining globally before any branch processing
• Enforce such limits on all branches which are processed

Instead we could:

• Allow each branch to have its own limits
• Evaluate them independently per branch

One thing to decide is what to do if a branch contains multiple upgrades and those upgrades have different limits set. I think we need to decided between the lowest limit (which could be "no limit") and the highest limit. I suggest we go with highest limits but log a warning if branches have mixed limits.

Something else to keep in mind is that we can "change hours" during a run, e.g. start at 10.59 for the first branch and process the second branch at 11.00, where hourly limits are reset.

Final point is that this relies on the results being returned by getPrList() being accurate/updated for all platforms. Issue created: #32040