Functional change: the leader scheduler is no longer implicitly
updated by PohRecorder via register_tick(). That's intended to
be a "feature" (crossing fingers).
Fullnode was the only real consumer of process_ledger and it was
only there to process a Blocktree. Blocktree is a tree, and a
ledger is a sequence, so something's clearly not right here.
Drop all other dependencies on process_ledger (only one test) so
that it can be fixed up in isolation.
