Make discarded MST blocks hang around for a bit?

[DavidBuchanan314]

Currently, we delete an MST block from the db the moment it's no longer needed for the current state of the db.

If someone is walking the MST via something like getBlocks, and the repo changes midway through, they'll get errors. If the repo is being updated rapidly, they'll probably never get a full read.

Rather than deleting them instantly we could tombstone them and garbage collect after some delay.