[iOS] Generate and send a turnstile event when map view event is queued

This change adds logic to push a turnstile event whenever a map view
event is queued. It guards against sending the turnstile event more
than once per session.

Note that `pushTurnstileEvent` is actually a "force push" in that it
flushes the queue and, since it enqueues a turnstile event, the
flush's call to `postEvents` will attempt to send any queued events
even if telemetry is paused. This has the side effect of defeating
the other queue / timing / flush logic in place whenever map load
(and now implicitly turnstile) events are pushed.

platform/ios/src/MGLMapboxEvents.m

@@ -275,11 +275,6 @@ - (instancetype) init {
              MGLMapboxEvents *strongSelf = weakSelf;
              [strongSelf validate];
          }];
-
-        // Turn the Mapbox Turnstile to Count App Users
-        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
-            [self pushTurnstileEvent];
-        });
     }
     return self;
 }
@@ -429,38 +424,42 @@ - (void)startUpdatingLocation {
     }
 }
 
+
 - (void) pushTurnstileEvent {
 
     __weak MGLMapboxEvents *weakSelf = self;
-
-    dispatch_async(_serialQueue, ^{
-
-        MGLMapboxEvents *strongSelf = weakSelf;
-
-        if ( ! strongSelf) return;
-
-        // Build only IDFV event
-        NSString *vid = [[[UIDevice currentDevice] identifierForVendor] UUIDString];
-
-        if (!vid) return;
-
-        NSDictionary *vevt = @{@"event" : MGLEventTypeAppUserTurnstile,
-                               @"created" : [strongSelf.rfc3339DateFormatter stringFromDate:[NSDate date]],
-                               @"appBundleId" : strongSelf.appBundleId,
-                               @"vendorId": vid,
-                               @"version": @(version),
-                               @"instance": strongSelf.instanceID};
-
-        // Add to Queue
-        [_eventQueue addObject:vevt];
-
-        // Flush
-        [strongSelf flush];
-
-        if ([strongSelf debugLoggingEnabled]) {
-            [strongSelf writeEventToLocalDebugLog:vevt];
-        }
-
+
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
+        dispatch_async(_serialQueue, ^{
+
+            MGLMapboxEvents *strongSelf = weakSelf;
+
+            if ( ! strongSelf) return;
+
+            // Build only IDFV event
+            NSString *vid = [[[UIDevice currentDevice] identifierForVendor] UUIDString];
+
+            if (!vid) return;
+
+            NSDictionary *vevt = @{@"event" : MGLEventTypeAppUserTurnstile,
+                                   @"created" : [strongSelf.rfc3339DateFormatter stringFromDate:[NSDate date]],
+                                   @"appBundleId" : strongSelf.appBundleId,
+                                   @"vendorId": vid,
+                                   @"version": @(version),
+                                   @"instance": strongSelf.instanceID};
+
+            // Add to Queue
+            [_eventQueue addObject:vevt];
+
+            // Flush
+            [strongSelf flush];
+
+            if ([strongSelf debugLoggingEnabled]) {
+                [strongSelf writeEventToLocalDebugLog:vevt];
+            }
+
+        });
     });
 }
 
@@ -484,13 +483,18 @@ - (void) pushEvent:(NSString *)event withAttributes:(MGLMapboxEventAttributes *)
         MGLMapboxEvents *strongSelf = weakSelf;
 
         if ( ! strongSelf) return;
+
+        if (!event) return;
 
+        // If it's a map load event then turn the Mapbox Turnstile to count app users
+        if ([event isEqualToString:MGLEventTypeMapLoad]) {
+            [self pushTurnstileEvent];
+        }
+
         // Metrics Collection Has Been Paused
         if (_paused) {
             return;
         }
-
-        if (!event) return;
 
         MGLMutableMapboxEventAttributes *evt = [MGLMutableMapboxEventAttributes dictionaryWithDictionary:attributeDictionary];
         // mapbox-events stock attributes
@@ -520,6 +524,7 @@ - (void) pushEvent:(NSString *)event withAttributes:(MGLMapboxEventAttributes *)
 
         // Put On The Queue
         [_eventQueue addObject:finalEvent];
+
 
         // Has Flush Limit Been Reached?
         if (_eventQueue.count >= MGLMaximumEventsPerFlush) {