Avoid deadlock

There are multiple methods which synchronize(self) in TSAccountMessage and also
multiple ones which can occur within a read-write transaction.

SignalServiceKit/src/Account/TSAccountManager.m

@@ -259,8 +259,11 @@ NSString *const TSAccountManager_NeedsAccountAttributesUpdateKey = @"TSAccountMa
 
 - (uint32_t)getOrGenerateRegistrationId:(YapDatabaseReadWriteTransaction *)transaction
 {
-    @synchronized(self)
+    // Unlike other methods in this class, there's no need for a `@synchronized` block
-    {
+    // here, since we're already in a write transaction, and all writes occur on a serial queue.
+    //
+    // Since other code in this class which uses @synchronized(self) also needs to open write
+    // transaction, using @synchronized(self) here, inside of a WriteTransaction risks deadlock.
     uint32_t registrationID = [[transaction objectForKey:TSAccountManager_LocalRegistrationIdKey
                                             inCollection:TSAccountManager_UserAccountCollection] unsignedIntValue];
 
@@ -273,7 +276,6 @@ NSString *const TSAccountManager_NeedsAccountAttributesUpdateKey = @"TSAccountMa
                   inCollection:TSAccountManager_UserAccountCollection];
     }
     return registrationID;
-    }
 }
 
 - (void)registerForPushNotificationsWithPushToken:(NSString *)pushToken