multihandlesink: Use the monotonic clock for detecting timeouts and connection durations

multihandlesink: Use the monotonic clock for detecting timeouts and connection durations

Otherwise real-time clock changes can wrongly trigger timeouts, or not
cause timeouts to happen in time.

Unfortunately real-time clock times still have to be kept track inside
the elements for the statistics. Switching those over to the monotonic
clock would cause behaviour changes from the application point of view.

The statistics are extended with fields with monotonic times though.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1137>