tenant_migration_donor_service.cpp and tenant_migration_recipient_service.cpp define overlapping sets of functions (with different capitalization of "if"):

• setPromiseOkIfNotReady
• setPromiseFromStatusIfNotReady
• setPromiseErrorIfNotReady

It's controversial whether we should test-and-set Promises like this at all. Let's clean up this mess. Don't factor them into future.h or future_util.h, because that would only encourage more usage of these functions, which are considered an anti-pattern. Put them in a tenant migration utility file, or eliminate their use entirely. The latter would be a project.