Remove an unnecessary splat in Future#traverse
Rewrite Future.first in the same style of .all
Fix a bug in Heap#delete

Deleting the last item did not actually delete it since it was replaced in the tree by itself.
Remove an unused constant
Make Unblocker#close more robust

When we’re shutting down we could be closing the unblocker and trying to unblock at the same time.
Fix a bug in Heap#delete

When deleting an item we need to bubble down as well as up, otherwise we’ll end up with parent nodes that are larger than their children.
Fix Timer#to_s
Version bump to v1.2.0.pre7