Keeping track of missing content

[morph-dev]

Idea is that each client can keep a record of content that should be, but it's missing from the network. It should also store the proof that content should be there (and some other information that is needed in order to generate such content outside of the protocol).

Details on how many content to keep and whether it should expire is up to debate.

This would be consumed by a bridge responsible for "healing" the network and potentially glados.