[WIP] Make panzer::DOFManager the only way we access dofs on elements

[bartgol]

This means that, inside cells, we will call the dof mgr method getElementGIDs and getElementLIDs to get a DOF's node lids/gids.

This should get rid of several arrays stored in the discretization, and (hopefully) make life easier for a) block discretizations and b) different mesh structs backends. The latter is intended for the upcoming Omega_h mesh struct, which will have to simply provide a ConnManager implementation, along the line of what's done in STKConnManager (which has been significantly trimmed down to match Albany's needs/uses).