Handle key collisions when stitching meshes

This triggered for me with a 16-bit dof_id_type buildbot test, but
there's no reason it couldn't happen with a larger id type and much
worse luck.

This isn't the only outstanding bug with that configuration, but it seems to be an independent and relatively simple problem so let's see what MooseBuild thinks of the fix.

It means that I only committed half a bugfix!  I copied the relevant code from find_neighbors, but didn't notice that the stitching code wasn't using a multimap.  Then the new code stopped throwing obvious failures, because missing neighbor pointers don't make other code scream the way wrong neighbor pointers do, and I started looking for the remaining bugs elsewhere.

Let me try switching to multimap and see what happens.