Fix potential race conditions while adding items to sync table

- The write operation dispatch queue is defined as concurrent, however,
the tasks that run in it are not thread safe.
- Changing dispatch queue type from concurrent to serial will take care
of it
- In the future, if needed, we can change it back to concurrent and
make the tasks that run in it thread safe.

sdk/iOS/src/MSSyncContext.m

@@ -53,7 +53,7 @@ -(id) initWithDelegate:(id<MSSyncContextDelegate>) delegate dataSource:(id<MSSyn
     self = [super init];
     if (self)
     {
-        writeOperationQueue = dispatch_queue_create("WriteOperationQueue", DISPATCH_QUEUE_CONCURRENT);
+        writeOperationQueue = dispatch_queue_create("WriteOperationQueue", DISPATCH_QUEUE_SERIAL);
 
         callbackQueue_ = callbackQueue;
         if (!callbackQueue_) {