This gets rid of the time.time() dependency, allowing the node to set
the chain time naturally using the generate RPC. Likewise this gets
rid of the arbitrary-block-numbering and auto-tip-movement system and
moves to a more direct form of tracking.
At the same time, we allow setup_clean_chain to be False, in which
case the generated coinbases will be smaller, so the funding needs to
be tweaked slightly.
Depends on D3601