[nit] `SyncUntil` does not mention in its name it only matches `key` on arrays

[ShadowJonathan]

Initially i thought that this function was generic enough to just match key and throw that variable into the check function, but turns out that's not the case.

I suggest renaming it to SyncUntilArray, and adding a generic function SyncUntil which does above expected behaviour.

[kegsay]

#272