Analyze & improve yagna event collecting

[johny-b]

What should yagna_event_collector do when collecting events fails?

Current logic is copied directly from yapapi PoolingBatch logic. Maybe it's good enough, but:

• should we have the same logic for batch-event-collecting and for other-event-collecting?
• implementation is ugly (e.g. imports private functions from yapapi)

(This is not in main yet: dd53787)