Revert "Revert "Merge remote-tracking branch 'urbit/master' into master""

This reverts commit dc6f72fa0a8ff2097b1e9c900ebec444c0d4c24f.
Get new event loop working with checkpoints

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.