systemclock: fixed race condition in handling alarms

systemclock: fixed race condition in handling alarms

When choosing the first entry from the list, gst_system_clock_async_thread
must set the entry state to busy before releasing the clock lock. Otherwise
a new entry could be added to the beginning of the list and
gst_system_clock_async_thread will be unaware and keep waiting on the entry
it has already chosen.

Also improved messages about expected state and bumped them to ERROR level
to detect unexpected state changes.

https://bugzilla.gnome.org/show_bug.cgi?id=760757