WIP: CMFActivity: remove 1k validation/queue/s rate limit

I discovered that CMFActivity's validation willingly hands control back to timerserver after a fixed number of activities were successfully validated (1000).

This means it is not possible to validate more than 1k activities per queue per second.

In this merge request, I introduce two preparatory changes and then the main logic change: let CMFActivity.distribute whether Activities.SQLBase.distribute reached the end of the activities it could try to validate (by returning on an empty select result), and only then hand control over to timerserver, similarly to what was already done for processing nodes.

/cc @Nicolas