Refactor BoundaryInfo sync and add elems

[lindsayad]

I'm doing this work in advance of potentially trying to build maps from higher-d elems, sides, and boundary meshes to lower-d elements

[moosebuild]

Job Coverage, step Generate coverage on 8f1eeb0 wanted to post the following:

Coverage

	eb5796	#4349 8f1eeb
	Total	Total	+/-	New
Rate	65.26%	64.48%	-0.78%	95.45%
Hits	77384	76449	-935	42
Misses	41195	42115	+920	2

Diff coverage report

Full coverage report

This comment will be updated on new commits.

[lindsayad]

will wait on #3759 before doing any more work