Submit Windowed PoSt in the Pipeline

[th7nder]

Scheduling the same partition

After Prove Commit we need to periodically prove partitions, not sectors...
If we schedule on prove commit sector 1 (partition 0),
and then sector 2 get assigned to the same partition 0, we cannot have 2 PoSt schedules running.

We need to keep track of the partition that is being proven periodically,
if we prove a sector and there is already a 'task pending' we don't schedule it.
so better be to decouple window PoSts from Prove Commit basically.

PoSt Proving must be assigned to a deadline and a partition.
We need to calculate when is the time when to send the proof - when the deadline opens.
Get the proving period, have the deadline index, have the proving window, have the offset.

After we register SP

We start schedule on registering the storage provider.
Every deadline we fetch partitions for this deadline and then perform submit windowed post.

1. How do we fetch partitions for a deadline, on-chain, or what?
2. How do we fetch partition's sectors?
3. Challenge for a partition is easy enough

Technicalities

1. Storage Provider Node will require PoSt verifying key on the start-up.
2. Expose deadline's partitions and their sectors via Storage Provider API trait.
3. Save proving_period_start in the Storage Provider Server.
4. Implement PoSt in the pipeline.
5. Schedule PoSt job for each deadline.
6. Schedule next PoSt jobs.
7. SubmitWindowedPoSt message only cares about deadlines, plus it should start prove commit on the challenge available? Or the deadline opens? Don't know when the stuff is added to the deadline.
8. Implement a function to query deadline info based on it's index.