Denominate payment rates in seconds rather than epochs.

[wjmelements]

Motivation

The time between blocks might change in the future.
This should not impact the duration of agreements or the payment rate over time.

If a future upgrade reduced the block time in half, that would double the rates of the payment rails.

prev: #204

[wjmelements]

Per discussion today we are targeting this for M4 @rjan90

[wjmelements]

This change will touch the other repos (filecoin-services and pdp) and cause breaking changes for synapse-sdk and curio.

[wjmelements]

FilOzone/filecoin-services#252 is somewhat important with this change, as otherwise we would be looping over timestamps which would be 30x worse

[wjmelements]

We will need to keep using epoch in some places. For example, PDP uses it for DRAND.

[wjmelements]

We will need to keep using epoch in some places. For example, PDP uses it for DRAND.

It is hardest to find a good way to convert between these two denominations that must both exist. It seems impossible to get a timestamp for a past epoch. I have thought about a permissionless mechanism for detecting that the time per block has changed, but it would not be able to confidently say when. Maybe a small bounty would incentivize someone to notify in a timely manner. Still not great.

As for the subdivision, here are the currently known constraints:

1. Payment rate should be denominated in seconds at the point where we multiply.
2. Proof challenge epochs must be future epochs at the point where we use them for beacon randomness.
3. FWS proving periods should be denominated in seconds

[Kubuxu]

That is roughly my conclusion as well. Durations can be denominated in seconds. Points in time have to be denominated in epochs.