Respond to CR.

// FREEBIE

src/Messages/OWSMessageSender.m

@@ -257,7 +257,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
 @property (nonatomic, readonly) id<ContactsManagerProtocol> contactsManager;
 @property (nonatomic, readonly) ContactsUpdater *contactsUpdater;
 @property (nonatomic, readonly) OWSDisappearingMessagesJob *disappearingMessagesJob;
-@property (atomic, readonly) NSMutableDictionary *sendingQueueMap;
+@property (atomic, readonly) NSMutableDictionary<NSString *, NSOperationQueue *> *sendingQueueMap;
 
 @end