More cleanup and sanitization

Moved constant values from ivars into constants. Invalidate the flush timer and clear the event queue on pause. Rewrote some flush logic to be easier to follow.

platform/ios/MGLMapboxEvents.m

@@ -44,6 +44,9 @@ NSString *const MGLEventGesturePanStart = @"Pan";
 NSString *const MGLEventGesturePinchStart = @"Pinch";
 NSString *const MGLEventGestureRotateStart = @"Rotation";
 
+const NSUInteger MGLMaximumEventsPerFlush = 20;
+const NSTimeInterval MGLFlushInterval = 60;
+
 @interface MGLMapboxEventsData : NSObject
 
 // All of the following properties are written to only from
@@ -140,7 +143,6 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
 @property (atomic) NSString *appName;
 @property (atomic) NSString *appVersion;
 @property (atomic) NSString *appBuildNumber;
-@property (atomic) NSUInteger flushAt;
 @property (atomic) NSDateFormatter *rfc3339DateFormatter;
 @property (atomic) NSURLSession *session;
 @property (atomic) NSData *digicertCert;
@@ -157,10 +159,6 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
 //
 @property (nonatomic) NSTimer *timer;
 
-// The flush expiration time is only ever accessed from the main thread.
-//
-@property (nonatomic) NSTimeInterval flushAfter;
-
 // This is an array of events to push. All access to it will be
 // from our own serial queue.
 //
@@ -241,8 +239,6 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
 
         // Events Control
         _eventQueue = [[NSMutableArray alloc] init];
-        _flushAt = 20;
-        _flushAfter = 60;
         
         // Setup Date Format
         _rfc3339DateFormatter = [[NSDateFormatter alloc] init];
@@ -298,6 +294,9 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
         return;
     }
     self.paused = YES;
+    [_timer invalidate];
+    _timer = nil;
+    [_eventQueue removeAllObjects];
     _data = nil;
     [_session invalidateAndCancel];
     _session = nil;
@@ -415,7 +414,7 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
         [_eventQueue addObject:finalEvent];
         
         // Has Flush Limit Been Reached?
-        if (_eventQueue.count >= strongSelf.flushAt) {
+        if (_eventQueue.count >= MGLMaximumEventsPerFlush) {
             [strongSelf flush];
         } else if (_eventQueue.count ==  1) {
             // If this is first new event on queue start timer,
@@ -439,12 +438,10 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
 
     dispatch_async(_serialQueue, ^{
         MGLMapboxEvents *strongSelf = weakSelf;
-        if ( ! strongSelf) return;
-        
-        __block NSArray *events;
+        if ( ! strongSelf || [_eventQueue count] == 0) return;
 
         // Make an immutable copy
-        events = [NSArray arrayWithArray:_eventQueue];
+        NSArray *events = [NSArray arrayWithArray:_eventQueue];
 
         // Update Queue to remove events sent to server
         [_eventQueue removeAllObjects];
@@ -494,14 +491,11 @@ NSString *const MGLEventGestureRotateStart = @"Rotation";
 - (void) startTimer {
     void (^timerBlock)() = ^{
         // Stop Timer if it already exists
-        if (_timer) {
-            [_timer invalidate];
-            _timer = nil;
-        }
+        [_timer invalidate];
 
         // Start New Timer
-        _timer = [NSTimer scheduledTimerWithTimeInterval:_flushAfter
-                                                  target:[self class]
+        _timer = [NSTimer scheduledTimerWithTimeInterval:MGLFlushInterval
+                                                  target:self
                                                 selector:@selector(flush)
                                                 userInfo:nil
                                                  repeats:YES];