[Proposal] Spending notes which haven't-yet been inserted

[spalladino]

@david-banks

Modify the current kernel implementation to forward all unmatched read_requests (those that read a “pending” new_commitment but have not been matched to one yet) to the next kernel iteration. Each kernel can then check its own read_requests as well as the previous kernel’s against all new_commitments seen thus far.

Doesn’t this suffer from the same issue as ordering public state updates, where A::foo could read C before even calling into A::bar?