Most of the external interface for PrimaryOnlyService has now been committed.

This ticket is to rehost the donor's logic onto a PrimaryOnlyService::TypedInstance.

See this unit test that has example of extending PrimaryOnlyService::TypedInstance.

CC spencer, matthew.russotto, ben.caimano

Edit: This ticket will need to create a class for the donor that extends PrimaryOnlyService::TypedInstance. We have also been talking about putting the donor's logic directly in the TenantMigrationAccessBlocker so that the migration's state does not need to be passed around in a 'donorStateDoc'. So, it would be convenient to use this ticket to move the donor's logic into TenantMigrationAccessBlocker (renaming it to "TenantMigrationDonor") and making this the class that extends PrimaryOnlyService::TypedInstance.