User: Bypass repeatable-read when creating an actor_id

User: Bypass repeatable-read when creating an actor_id

When MySQL is using repeatable-read transaction isolation (which is the
default), the following sequence of events can occur:

1. Request A: Begin a transaction.
2. Request A: Try to select the actor ID for a user. Find no rows.
3. Request B: Insert an actor ID for that user.
4. Request A: Try to insert an actor ID for the user. Fails because one
   exists.
5. Request A: Try to select the actor ID that must exist. Fail because of
   the snapshot created at step 2.

In MySQL we can avoid this issue at step #5 by using a locking select
(FOR UPDATE or LOCK IN SHARE MODE), so let's do that.

Bug: T210621
Change-Id: I6c1d255fdd14c6f49d2ea9790e7bd7d101e98ee4