Move 21 durability-use-next methods plus DurabilityUseNextSession
and DurabilityCandidate verbatim into MacroDurability. Only outside
references are isCurrentRunActive, callOnClientThread and
waitOneTick (already package-visible), qualified as MacroExecutor.xxx
/ MacroDurability.xxx at the 2 staying call sites. All bodies
verified identical against HEAD modulo that qualification.
