(streaming) validator fees

[Ivshti]

Implement validator fees that will be calculated based on # of processed events.

For the publisher side, this adds more incentive to forge events, but the same security guarantees apply (they're not the state leader)

For the advertiser side, incentives are OK cause they lose out more from forging events (impression events pay to the publisher).

So if the payment spec defines fees per IMPRESSION event, pay them out - that'd be pretty simple.

[Ivshti]

done in #63