Modifying u2_reck is a bad idea in the face of checkpoints. Just stick
everything on roe. This actually cleans up the logic a bit -- we can
execute non-core-modifying actions immediately, rather than on the next
tick.
Carts are now batches of events waiting to be or just recently
persisted. _lo_work calls u2_reck_kick on the last batch of events,
removes every batch that's been through Raft, and then prepares the next
batch.
TODO make _lo_punk et al stick things on roe
TODO move disk persistence to raft.c
TODO raft_push: lo_open / mark / lo_shut
