netclientclock: Destroy a cached clock 60 seconds after its last use
netclientclock: Destroy a cached clock 60 seconds after its last use

There's not much lost by having the clock idle around a bit longer but it will
potentially allow anybody wanting to use the same clock server again to sync
much faster.

StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer