Multiple heads protection: switch to Heptapod flag and enforce with bookmarks
First we complete the switch from experimental.single-head-per-branch
to heptapod.allow-multiple-heads
that was started in Heptapod 0.17.2: the latter does not imply the former any more, and we don't bypass the former to allow the double evaluation.
Then this allows us to keep the multiple heads protection done exclusively in py-heptapod, even when there are bookmarks.