add a second rendezvous point

This is currently unused, but we now have a second, independent rendezvous point
available. The long term idea here is to have all threads rendezvous in
exit_with() using this second point, *after* opting out of the first rendezvous
point. After they've synchronised using this, they can go on in a next phase to
complete liveness checks. This ensures no threads overlap exploration and
liveness checking which avoids some messy scenarios. E.g. one thread wanting to
expand and migrate the seen set while another is probing its liveness. The delay
at the second rendezvous point should be acceptable because all threads should
be noticing someone has flipped the done flag and proceeding to a timely exit
anyway.

Github: related to #47 "properties, covers, etc"