Too many "deprecation" will be hidden, we should probably live with some
warning about backward compatibility.

Signed-off-by: moznion <[email protected]>

And make a suggestion to use interfaces that are in the `core` package.

Signed-off-by: moznion <[email protected]>
Also-by: Simon Bernard <[email protected]>

To call each interface method on corresponded `LwM2mObjectTree` method.

Signed-off-by: moznion <[email protected]>

And call each interface's method at related method of `LeshanClient`;
i.e.  `start()`, `stop()` and `destroy()`.

Signed-off-by: moznion <[email protected]>

This interface extends `LwM2mClientObserver` interface.
And make `LwM2mClientObserverAdapter` and
`LwM2mClientObserverDispatcher`
implement that new interface.
This doesn't break the backward compatibility because
`LwM2mClientObserver2` is compatible with `LwM2mClientObserver` and each
implementing class conceals the difference between `LwM2mClientObserver`
and `LwM2mClientObserver2`.

Signed-off-by: moznion <[email protected]>

If a task that are belong to `DefaultRegistrationEngine` raises
unexpected `RuntimeException` and the `observer` member variable
implements `LwM2mClientObserver2` (instead of `LwM2mClientObserver`),
it calls `LwM2mClientObserver2#onUnexpectedError()` hook.
The purpose of this hook gimmick is to shutdown the client application
mainly.

Signed-off-by: moznion <[email protected]>

Signed-off-by: moznion <[email protected]>
Also-by: Simon Bernard <[email protected]>