Use the new MeshBase::get_boundary_info() API in libmesh.

Refs libMesh/libmesh#388.
Reduce the number of times we call get_boundary_info().

Generally clean up some code.

Refs libMesh/libmesh#388.
Merge pull request #4122 from jwpeterson/boundary_info

Use the new get_boundary_info() API in libmesh