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
(cherry picked from commit
37f48fdb25a78ba7623c57b50cdfd842292d3ccb)