rework refactor _aspiringchildren by introducing _possibledestination This allows us to reuse some of the logic for evolve from _aspiringchildren for the new implementation of evolve --all. The logic is also better as some previously selected changesets may not actually evolve on the target, and some changesets that does not would not.

test-drop.t
test-simple4server.t
tests/test-simple4server-bundle2.t