Configurable max sector count per partition for WindowPoSt

[hyunmoon]

https://github.com/filecoin-project/specs-actors/blob/d5a64ed5625c2ef2cd484de6797b2d00efcc204b/actors/abi/sector.go#L134-L141

Currently WindowPoStPartitionSectors is a fixed constant of 2349 for a 32GiB sector miner.
This means a miner will be submitting a WindowPoSt only once a day up to about 75TiB.

This may help save chain bandwidth but for small to medium sized miners, a failure to submit a single WindowPoSt results in total loss of their power for 24 hours.

I feel the current proof-of-spacetime design requires too much rigor to average miners.
Allowing them to spread out the sectors over multiple deadlines as they wish will help them avoid being overly punished.
If this decision is indeed intentional for saving gas fees, please find a way to make WindowPoSt submission more affordable.

[anorth]

Thanks for the issue. There's a trade-off between requiring high system uptime if PoSts are spread out, and the risk associated with a single failure. Packing consecutive sector numbers into one partition is also far better for chain state since, since the associated bitfield compresses very well. And, yes, chain message bandwidth is definitely a consideration.

We could alter the allocation logic to fill chunks smaller than a whole partition, paying some state size and message throughput for a less sharp failure cost. I think we'll need to reach a chain bandwidth surplus first, though.