Break retain cycles

These were always present, but weren't obvious due to obj-c implicitly
retaining self (which there's no an warning enabled to catch.)

Author: rsgowman

Firestore/Source/Remote/FSTRemoteStore.mm

@@ -33,6 +33,7 @@
 #import "Firestore/Source/Remote/FSTStream.h"
 #import "Firestore/Source/Remote/FSTWatchChange.h"
 #import "Firestore/Source/Util/FSTAssert.h"
+#import "Firestore/Source/Util/FSTClasses.h"
 #import "Firestore/Source/Util/FSTLogger.h"
 
 #include "Firestore/core/src/firebase/firestore/auth/user.h"
@@ -503,8 +504,13 @@ - (void)processBatchedWatchChanges:(NSArray<FSTWatchChange *> *)changes
 
   // Update in-memory resume tokens. FSTLocalStore will update the persistent view of these when
   // applying the completed FSTRemoteEvent.
+  FSTWeakify(self);
   [remoteEvent.targetChanges enumerateKeysAndObjectsUsingBlock:^(
                                  FSTBoxedTargetID *target, FSTTargetChange *change, BOOL *stop) {
+    FSTStrongify(self);
+    if (!self) {
+      return;
+    }
     NSData *resumeToken = change.resumeToken;
     if (resumeToken.length > 0) {
       FSTQueryData *queryData = self->_listenTargets[target];

Firestore/Source/Util/FSTAsyncQueryListener.mm

@@ -16,6 +16,7 @@
 
 #import "Firestore/Source/Util/FSTAsyncQueryListener.h"
 
+#import "Firestore/Source/Util/FSTClasses.h"
 #import "Firestore/Source/Util/FSTDispatchQueue.h"
 
 @implementation FSTAsyncQueryListener {
@@ -34,8 +35,18 @@ - (instancetype)initWithDispatchQueue:(FSTDispatchQueue *)dispatchQueue
 }
 
 - (FSTViewSnapshotHandler)asyncSnapshotHandler {
+  FSTWeakify(self);
   return ^(FSTViewSnapshot *_Nullable snapshot, NSError *_Nullable error) {
+    FSTStrongify(self);
+    if (!self) {
+      return;
+    }
+    FSTWeakify(self);
     [self->_dispatchQueue dispatchAsync:^{
+      FSTStrongify(self);
+      if (!self) {
+        return;
+      }
       if (!self->_muted) {
         self->_snapshotHandler(snapshot, error);
       }