checkpoint: rename temporary types to ephemeral vocabulary

Type-level rename of the shadow-branch surface (method names and the generic Read/Write collapse follow): TemporaryStore -> EphemeralStore WriteTemporaryOptions -> WriteEphemeralOptions WriteTemporaryResult -> WriteEphemeralResult WriteTemporaryTaskOptions -> WriteEphemeralTaskOptions ReadTemporaryResult -> ReadEphemeralResult TemporaryInfo -> EphemeralInfo TemporaryCheckpointInfo -> EphemeralCheckpointInfo