Rework flush of registration connection(s).

SignalServiceKit/src/Storage/OWSPrimaryStorage.m

@@ -138,6 +138,7 @@ void runAsyncRegistrationsForStorage(OWSStorage *storage)
 
     DDLogVerbose(@"%@ async registrations enqueued.", self.logTag);
 
+    [[self registrationConnection] asyncReadWriteWithBlock:^(YapDatabaseReadWriteTransaction *transaction) {
         // Block until all async registrations are complete.
         //
         // NOTE: This has to happen on the "registration connections" for this
@@ -147,12 +148,12 @@ void runAsyncRegistrationsForStorage(OWSStorage *storage)
         DDLogVerbose(@"%@ flushing registration connections: %zd.", self.logTag, pendingRegistrationConnectionSet.count);
 
         dispatch_async(dispatch_get_main_queue(), ^{
+            if (pendingRegistrationConnectionSet.count > 0) {
                 for (YapDatabaseConnection *dbConnection in pendingRegistrationConnectionSet) {
                     [dbConnection
                         flushTransactionsWithCompletionQueue:dispatch_get_main_queue()
                                              completionBlock:^{
                                                  OWSAssertIsOnMainThread();
-                                         OWSAssert(!self.areAsyncRegistrationsComplete);
 
                                                  [pendingRegistrationConnectionSet removeObject:dbConnection];
                                                  if (pendingRegistrationConnectionSet.count > 0) {
@@ -160,14 +161,27 @@ void runAsyncRegistrationsForStorage(OWSStorage *storage)
                                                      return;
                                                  }
 
+                                                 [self markAsyncRegistrationsAsCompleteWithCompletion:completion];
+                                             }];
+                }
+            } else {
+                [self markAsyncRegistrationsAsCompleteWithCompletion:completion];
+            }
+        });
+    }];
+}
+
+- (void)markAsyncRegistrationsAsCompleteWithCompletion:(void (^_Nonnull)(void))completion
+{
+    OWSAssertIsOnMainThread();
+    OWSAssert(!self.areAsyncRegistrationsComplete);
+    OWSAssert(completion);
+
     DDLogVerbose(@"%@ async registrations complete.", self.logTag);
 
     self.areAsyncRegistrationsComplete = YES;
 
     completion();
-                                     }];
-        }
-    });
 }
 
 + (void)protectFiles