Small improvements to migration script
- wait for slaves after every batch in migrateCentralWiki.php
- avoid PHP notice due DBConnRef objects not going through MWOAuthUtils::getCentralDB. Migration should be done in readonly mode anyway, so ignoring that for the target DB is fine.
- don't use "migrating grant" text when migrating consumers
Bug: T59336
Change-Id: I9c52534970bb04175b909f41846e83f1aef88d44