fix(database): fix race condition for iOS

The exception raised is 'createRepo called for Repo that already exists.'
in FRepoManager.m
It looks like we have to call the firebase database reference from the same
thread/queue otherwise we have a race condition leading to that exception.

ios/RNFirebase/database/RNFirebaseDatabase.m

@@ -96,11 +96,11 @@ - (id)init {
                   path:(NSString *)path
                   transactionId:(nonnull NSNumber *)transactionId
                   applyLocally:(BOOL)applyLocally) {
+    FIRDatabaseReference *ref = [self getReferenceForAppPath:appDisplayName dbURL:dbURL path:path];
     dispatch_async(_transactionQueue, ^{
         NSMutableDictionary *transactionState = [NSMutableDictionary new];
         dispatch_semaphore_t sema = dispatch_semaphore_create(0);
         transactionState[@"semaphore"] = sema;
-        FIRDatabaseReference *ref = [self getReferenceForAppPath:appDisplayName dbURL:dbURL path:path];
 
         [ref runTransactionBlock:^FIRTransactionResult *_Nonnull (FIRMutableData *_Nonnull currentData) {
             dispatch_barrier_async(_transactionQueue, ^{