netclientclock: Make the clock a wrapper clock around an internal clock

netclientclock: Make the clock a wrapper clock around an internal clock

The internal clock is only used for slaving against the remote clock, while
the user-facing GstClock can be additionally slaved to another clock if
desired. By default, if no master clock is set, this has exactly the same
behaviour as before. If a master clock is set (which was not allowed before),
the user-facing clock is reporting the remote clock as internal time and
slaves this to the master clock.

This also removes the weirdness that the internal time of the netclientclock
was always the system clock time, and not the remote clock time.

https://bugzilla.gnome.org/show_bug.cgi?id=750574