Fix implicit retain self warnings (firebase#808)

Xcode has starting warning about us implicitly retaining self references
within blocks. This commit fixes it by explicitly mentioning self. No
real changes are introduced here; this is effectively just making
implicit behaviour explicit.

Author: rsgowman

Firestore/Source/Auth/FSTCredentialsProvider.mm

@@ -86,12 +86,12 @@ - (instancetype)initWithApp:(FIRApp *)app {
 
                       NSString *userID = userInfo[FIRAuthStateDidChangeInternalNotificationUIDKey];
                       User newUser = User(userID);
-                      if (newUser != _currentUser) {
-                        _currentUser = newUser;
+                      if (newUser != self->_currentUser) {
+                        self->_currentUser = newUser;
                         self.userCounter++;
                         FSTVoidUserBlock listenerBlock = self.userChangeListener;
                         if (listenerBlock) {
-                          listenerBlock(_currentUser);
+                          listenerBlock(self->_currentUser);
                         }
                       }
                     }
@@ -121,7 +121,7 @@ - (void)getTokenForcingRefresh:(BOOL)forceRefresh
                                                    userInfo:errorInfo];
             completion(Token::Invalid(), cancelError);
           } else {
-            completion(Token(util::MakeStringView(token), _currentUser), error);
+            completion(Token(util::MakeStringView(token), self->_currentUser), error);
           }
         };
       };

Firestore/Source/Remote/FSTRemoteStore.mm

@@ -507,10 +507,10 @@ - (void)processBatchedWatchChanges:(NSArray<FSTWatchChange *> *)changes
                                  FSTBoxedTargetID *target, FSTTargetChange *change, BOOL *stop) {
     NSData *resumeToken = change.resumeToken;
     if (resumeToken.length > 0) {
-      FSTQueryData *queryData = _listenTargets[target];
+      FSTQueryData *queryData = self->_listenTargets[target];
       // A watched target might have been removed already.
       if (queryData) {
-        _listenTargets[target] =
+        self->_listenTargets[target] =
             [queryData queryDataByReplacingSnapshotVersion:change.snapshotVersion
                                                resumeToken:resumeToken];
       }

Firestore/Source/Util/FSTAsyncQueryListener.mm

@@ -34,10 +34,15 @@ - (instancetype)initWithDispatchQueue:(FSTDispatchQueue *)dispatchQueue
 }
 
 - (FSTViewSnapshotHandler)asyncSnapshotHandler {
+  // Retain `self` strongly in resulting snapshot handler so that even if the
+  // user releases the `FSTAsyncQueryListener` we'll continue to deliver
+  // events. This is done specifically to facilitate the common case where
+  // users just want to turn on notifications "forever" and don't want to have
+  // to keep track of our handle to keep them going.
   return ^(FSTViewSnapshot *_Nullable snapshot, NSError *_Nullable error) {
-    [_dispatchQueue dispatchAsync:^{
-      if (!_muted) {
-        _snapshotHandler(snapshot, error);
+    [self->_dispatchQueue dispatchAsync:^{
+      if (!self->_muted) {
+        self->_snapshotHandler(snapshot, error);
       }
     }];
   };