Add ability to expire all trees

Add the ability to easily expire all hashtrees just like they can easily
be cleared. This may be useful sometime in production if you don't trust
the hashtrees for some reason and is also useful for testing.
Add test to verify that expiration does not prevent exchange

Make sure that when trees are expired it does not prevent them from
exchanging. The entire purpose of expiration is to provide behavior
similar to clear except that exchange can occur up until the point of
tree rebuilding.