Path db

In short, moves the currentPath structure in consensus to disk, and fixes associated bugs.

Proposal for error handling: panic on read, and error catch on writes. We do a fair amount of verification when modifying the structures, so it makes sense to use this existing structure. Ideally, database inconsistencies would be caught on write where they can be handled more elegantly (e.g. rejecting a call to acceptBlock).

Also worth noting that this is on top of the Set db pull request.