clock: Add GST_CLOCK_FLAG_NEEDS_STARTUP_SYNC and related API

clock: Add GST_CLOCK_FLAG_NEEDS_STARTUP_SYNC and related API

gst_clock_wait_for_sync(), gst_clock_is_synced() and gst_clock_set_synced()
plus a signal to asynchronously wait for the clock to be synced.

This can be used by clocks to signal that they need initial synchronization
before they can report any time, and that this synchronization can also get
completely lost at some point. Network clocks, like the GStreamer
netclientclock, NTP or PTP clocks are examples for clocks where this is useful
to have as they can't report any time at all before they're synced.

https://bugzilla.gnome.org/show_bug.cgi?id=749391