Made the prior user existence check in LoginForm use DB_MASTER

Made the prior user existence check in LoginForm use DB_MASTER

* This helps if multiple account creation attempts were made in a row
  and the slave selected was lagged (either too much for Chronology
  Protector or the user did not resend the cookies to link the session).
* The locking also better handles concurrent attempt to make another
  account, especially with CentralAuth trying to make external accounts.
  This assumes that the rate of concurrent account creations with close
  names is low enough given trx speed to avoid gap locking issues. This
  will need to be confirmed with low error log rates to be sure.
* The User::idFromName() method now includes a $flags parameter.

Bug: T104615
Change-Id: I8385526a19efc528a016ad2bbf376b377138966b