Cleaner initialization of shared structures in ParMesh

[najlkin]

This PR was separated from #4128 . It moves the initialization of the structures for shared edges (sedge_ledge, sface_lface) through FinalizeParTopo() from Finalize() directly to the methods allocating these structures, which were leaving them uninitialized before. This makes the code cleaner, better maintainable and less prone to bugs 😉 . However, it poses a slight risk if someone uses in a very-nonstandard way or overrides the protected methods and Finalize directly. This also avoids double update of these structures in Load() unless necessary.