Add functionality to Boost to monitor and extend `Available` sectors for SnapDeals

[nonsense]

MaxSectorExpirationExtension is 540 days.

Many storage clients are trying to make deals with a duration of >520 days, with start epoch 2-3 days from the current epoch.

This means that an SP that wants to do SnapDeals must always have a set of fresh sectors that were extended "recently" -- in the last few days.

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We should have functionality in Boost that monitors the number of fresh sectors, and provides an easy way to extend them periodically.

The set of sectors that need to be kept fresh like this is related to the max rate of snap dealing which possibly makes it a tiny fraction of total CC sectors.

[ZenGround0]

The fresh requirement can be a constraint the lotus-miner imposes on all sectors marked as ready for snap-deal upgrade. It might make more sense on lotus-miner because I don't think boost has access to the sealer's internal data on which sectors are marked as ready for snap deals. But if I'm wrong about this or if we make this info available to boost then it could make more sense to do the keep fresh thing over here.