Rework flush of registration connection(s).

SignalServiceKit/src/Storage/OWSPrimaryStorage.m

@@ -138,14 +138,19 @@ void runAsyncRegistrationsForStorage(OWSStorage *storage)
 
     DDLogVerbose(@"%@ async registrations enqueued.", self.logTag);
 
-    [[self registrationConnection] asyncReadWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
+    // Flush the write queue to ensure all async registrations have begun.
+    [[self registrationConnection]
+        flushTransactionsWithCompletionQueue:dispatch_get_main_queue()
+                             completionBlock:^{
                                  // Block until all async registrations are complete.
                                  //
                                  // NOTE: This has to happen on the "registration connections" for this
                                  //       database.
-        NSMutableSet<YapDatabaseConnection *> *pendingRegistrationConnectionSet =
-            [[((OWSDatabase *)self.database)clearCollectedRegistrationConnections] mutableCopy];
-        DDLogVerbose(@"%@ flushing registration connections: %zd.", self.logTag, pendingRegistrationConnectionSet.count);
+                                 NSMutableSet<YapDatabaseConnection *> *pendingRegistrationConnectionSet = [
+                                     [((OWSDatabase *)self.database)clearCollectedRegistrationConnections] mutableCopy];
+                                 DDLogVerbose(@"%@ flushing registration connections: %zd.",
+                                     self.logTag,
+                                     pendingRegistrationConnectionSet.count);
 
                                  dispatch_async(dispatch_get_main_queue(), ^{
                                      if (pendingRegistrationConnectionSet.count > 0) {
@@ -155,13 +160,19 @@ void runAsyncRegistrationsForStorage(OWSStorage *storage)
                                                                       completionBlock:^{
                                                                           OWSAssertIsOnMainThread();
 
-                                                 [pendingRegistrationConnectionSet removeObject:dbConnection];
-                                                 if (pendingRegistrationConnectionSet.count > 0) {
-                                                     DDLogVerbose(@"%@ registration connection flushed.", self.logTag);
+                                                                          [pendingRegistrationConnectionSet
+                                                                              removeObject:dbConnection];
+                                                                          if (pendingRegistrationConnectionSet.count
+                                                                              > 0) {
+                                                                              DDLogVerbose(@"%@ registration "
+                                                                                           @"connection flushed.",
+                                                                                  self.logTag);
                                                                               return;
                                                                           }
 
-                                                 [self markAsyncRegistrationsAsCompleteWithCompletion:completion];
+                                                                          [self
+                                                                              markAsyncRegistrationsAsCompleteWithCompletion:
+                                                                                  completion];
                                                                       }];
                                          }
                                      } else {