`set`/`append` side channel implementation mitigations

[yoavweiss]

Given that set and append are synchronous, they could create a side channel exposing if a value is currently set or not, at least if naively implemented. (e.g. if setting a value takes different times based on its presence or lack-there-of)
I think it's worthwhile to point that out (e.g. in a note), to ensure implementations are performing the actual setting/appending in an async fashion.