Sector Expiration Fix

[ZenGround0]

#836 rushed through a sector expiration change that doesn't seem exactly right. I think I should have changed max sector lifetime and instead changed max extension

1. SectorMaximumLifetime() should be changed to 540 days from five years
2. MaxSectorExpirationExtension should move back to 366.

@zixuanzh @nicola let me know if this sounds alright with you. This doesn't matter for incentinet since it's not going to last 180 days.

@anorth FYI

[anorth]

I don't think any change is needed. We should not imply the arithmetic around subtracting 180 from 540. We should set this value based on any independent restriction that exists.

@zixuanzh's comments explicitly say there is no restriction. We could set this to max uint64 to be clear about that. Otherwise set it to equal max sector lifetime ?

[zixuanzh]

@nicola raised that there might be a need for MinSectorLifetimeExtension.

It seems reasonable to set that to the MinSectorLifetime. In other words, given a MinSectorLifetime of 180 days and MaxSectorLifetime of 540 days. Sectors can be extended at minimum 180 days at a time up till the maximum SectorLifetime (540 days) since the StartEpoch of the original sector. There is no independent restriction for MaxSectorLifetimeExtension but the practical bound is 540 - 180.

[anorth]

I'll await the promotion of "might" to "must".