Cleanup source.dynamic, fix append

[toots]

This PR does some cleanup of source.dynamic after debugging append:

In particular, it removes the set method and resurection_time parameter.

resurection_time was never used in the code base and creates weird issues when passing sources that are less long than the given time.

I believe that a good general rule is that source.dynamic is powerful and the user should be careful to control the kind of issues that resurection_time was supposed to automatically take care of.

The semantics for set is not clear: does it change the source immediately? Following the track_sensitive param?

Its effect can always be implemented using the next source callback, which is what that PR does.

It also fixes a bug: sources should be put to sleep after being used in source.dynamic as they can be re-used elsewhere or later in the operator.

A convenience methods is added that makes it possible to prepare a source before it is given to source.dynamic, for instance when creating it at the beginning of a track and returning it at its end.

This makes it possible to fetch request, etc. to have the maximum chance of being ready when the source is used.

merge is added as a getter, making it possible to selectively merge tracks when switching sources.

The operator is also no longer marked experimental. It supports all the single and more at this point so it makes sens to give it more visibility.

Lastly, append is rewritten and fixed using these new APIs.