Integrate new Message API with communication endpoints.

- Revert some changes from a previous commit, that may be envisioned
  again when revising APIs in-depth for DecLearn 3.0.
- Stitch back the 'communication' and 'messaging' submodules together,
  having 'NetworkClient' and 'NetworkServer' send 'Message' instances
  (that are merely serialized to string prior to being exchanged) and
  parse received strings into 'SerializedMessage' instances.
- This change is a compromise between keeping things as before (which
  would not benefit from the introduced delayed-parsing capability of
  'SerializedMessage'), and changing them so deeply that current code
  making use of communication endpoint would require heavy revisions.