feat(piecesScheduledRemove): Remove Piece Metadata

[wjmelements]

Reviewer @rvagg
This reduces storage accumulation.
However the piece metadata will still not be removed when the whole data set is deleted.

Changes

• rm piece metadata in piecesScheduledRemove

[rvagg]

There's a missing window here though, piece won't actually be removed from data set until the nextProvingPeriod is called on that data set. So now we have a piece with no metadata - user, or others, will list pieces in a data set, see them, but with nothing about them.

Not a great state of affairs. Would it be practical to extend PDPListener#nextProvingPeriod to pass through uint256[] removals? I wonder if that could get too big to pass as an argument there...

[rvagg]

We're going to need some state for this I think, I don't see a good way around this that doesn't involve iterating over pieces during nextProvingPeriod. It'll probably have to mirror PDPVerifier's:

    // Enqueued piece ids for removal when starting the next proving period
    mapping(uint256 => uint256[]) scheduledRemovals;

And we update it when we get a call to piecesScheduledRemove. Then during nextProvingPeriod we can perform the metadata clean-up and purge that list.

[rvagg]

Moving this to M4, we should try and get this landed, but it needs to be done in a queue to be processed at nextProvingPeriod and since we have more state, we need to be careful wrt upgrading existing contracts.