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.
Wait, what does this even mean for a map?

http://www.cplusplus.com/reference/map/map/equal_range/

"Because the elements in a map container have unique keys, the range returned will contain a single element at most."