Integrate pdp updates -- nextProvingPeriod called after first add

[ZenGround0]

This is an attempt to bring curio up to date with the latest changes here: FilOzone/pdp#86. The main difference is that now pdp add does not register a challenge generation, that must happen explicitly through a call to nextprovingperiod.

The approach here is

1. Create a new harmony task just for initializing proof sets with first call to nextProvingPeriod
2. Differentiate these jobs from regular nextProvingPeriod jobs by checking that prove_at_epoch IS NULL
3. initProvingPeriod jobs also guarded on new proofset schema column boolean flag init_ready being set to true, which happens on the first add

[magik6k]

Reads correctly, so if it works should be fine to merge

[ZenGround0]

Ok @magik6k I've finally got this consistently proving. Please give it another review.