specify the behavior of the ProvingPeriodDuration function

[laser]

The spec shows that the SubmitPoSt and CommitSector methods rely on a function called ProvingPeriodDuration which produces the number of blocks in a proving period given a sector size.

We need to understand how to produce the number of blocks in a proving period given a sector size.

[mhammersley]

@sternhenri for awareness

[sternhenri]

Sorry for lag. @whyrusleeping, @dignifiedquire or @nicola, can you explain how a provingPeriod is calculated for a given sector size?

[anorth]

Quoth @nicola in slack

proving period so far is the same for all sector size, the minimum sector size will probably be 64GB and we will pick the proving period for 64GB, the generation attack time for larger sectors will always be larger than those of 64GB

in other words, by picking a good proving period for the smallest sector, we ensure a secure proving period for all sectors larger than that

(We still need an actual number to work with, of course)