Deprecated (undocumented) g_thread_gettime

Deprecated (undocumented) g_thread_gettime

g_thread_gettime() is an undocumented public function pointer that
points to a function that returns the monotonic time in nanoseconds.
g_get_monotonic_time() does the same in microseconds, so it can be used
instead.

GLib had one internal user in GFileMonitor that only cared about
millisecond accuracy; it has been ported to g_get_monotonic_time().