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.
Switching to multimap fixes misc_ex10 with 16-bit dof_id_type.  I'll put that in a separate PR just to be sure I didn't break anything else in the process.