Delay GDTCORUploadCoordinator timer initial run

If an upload coordinator gets created during `+load` calls, it can end up running its immediately, while +load calls are still running. It's generally a bad idea to be splitting off threads while +load calls are still running.

The upload coordinator checks every 30 seconds, and doesn't really need to run right away. Since it's easy to transitively create an upload coordinator from other calls that happen during +load, we'll just push off the initial run of the timer slightly. That makes it less likely to interfere (or deadlock) with other load calls.

See firebase#7171 for an example of a problem solved by this change.

GoogleDataTransport/GDTCORLibrary/GDTCORUploadCoordinator.m

@@ -68,9 +68,12 @@ - (void)startTimer {
       return;
     }
 
+    // Delay the timer slightly so it doesn't run while +load calls are still running.
+    dispatch_time_t deadline = dispatch_time(DISPATCH_TIME_NOW, 1 * NSEC_PER_SEC);
+
     self->_timer =
         dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, self->_coordinationQueue);
-    dispatch_source_set_timer(self->_timer, DISPATCH_TIME_NOW, self->_timerInterval,
+    dispatch_source_set_timer(self->_timer, deadline, self->_timerInterval,
                               self->_timerLeeway);
 
     dispatch_source_set_event_handler(self->_timer, ^{