Fix deadlock when responding to contacts sync messages.

SignalServiceKit/src/Messages/OWSMessageManager.m

@@ -606,6 +606,13 @@ NS_ASSUME_NONNULL_BEGIN
         }
     } else if (syncMessage.hasRequest) {
         if (syncMessage.request.type == OWSSignalServiceProtosSyncMessageRequestTypeContacts) {
+            // We respond asynchronously because populating the sync message will
+            // create transactions and it's not practical (due to locking in the OWSIdentityManager)
+            // to plumb our transaction through.
+            //
+            // In rare cases this means we won't respond to the sync request, but that's
+            // acceptable.
+            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                 OWSSyncContactsMessage *syncContactsMessage =
                     [[OWSSyncContactsMessage alloc] initWithSignalAccounts:self.contactsManager.signalAccounts
                                                            identityManager:self.identityManager
@@ -619,8 +626,10 @@ NS_ASSUME_NONNULL_BEGIN
                         DDLogInfo(@"%@ Successfully sent Contacts response syncMessage.", self.logTag);
                     }
                     failure:^(NSError *error) {
-                    DDLogError(@"%@ Failed to send Contacts response syncMessage with error: %@", self.logTag, error);
+                        DDLogError(
+                            @"%@ Failed to send Contacts response syncMessage with error: %@", self.logTag, error);
                     }];
+            });
         } else if (syncMessage.request.type == OWSSignalServiceProtosSyncMessageRequestTypeGroups) {
             OWSSyncGroupsMessage *syncGroupsMessage = [[OWSSyncGroupsMessage alloc] init];
             DataSource *dataSource = [DataSourceValue