Fix PBC issues for good (hopefully) #2703
Description
The periodic boundary condition system in libmesh has some issues that are difficult to replicate: an example is
#2590 - "I'm hurting now to look back at the optimism I had to just create a PR that I thought would fix everything so an issue wasn't even necessary..."
After many tries @roystgnr decided that patching code doesn't seem to be a good option (see #2700).
Issue #2590 is going to be the reference PR for this Issue.
@roystgnr feel free to edit this by writing down all the issues you think we might have and that this PR will resolve.
I will help to prepare test cases (unfortunately in MOOSE) and meshes to make sure we have good coverage.
Issue list:
- with fake neighbors (e.g. neighbors physically disconnected but with a neighbor pointer and overlapping nodes) PBC results are not correct
- corner cases where multiple PBC meet in distributed mode (Fix distributed periodic constraints bug #2590)