Abstract methods overriden by subclass.

* TransportLayer is never instantiated, so all its overriden methods
  are dead code, since the subclass never refers to them.
Remove unused methods (iteration 1).
Remove unused methods (iteration 2).
Remove unused methods (iteration 3).
Remove unused methods (iteration 4) and imports.
Remove unused uri_parts.

* Also remove tests.
Remove unused code from protocol module.
Remove tests of dropped functions.
Various cleanups.

* Move `store` function to CryptoTransportLayer; TransportLayer has no
  `dht` attribute.
* Drop getters for public attributes.
* Clarify code using `get`.
* Use None as default callback.
* Minor style fixes.
Merge pull request #828 from yagoulas/thin_transport2

Remove dead code from transport.py