* move kv helpers to helpers_kv*.go
* change KVGetJSON return signature
Return a boolean and an error, to clearly indicate if no value was found and thus no value unmarshalled into the target interface.
Also fix an issue with CompareAndSet to allow an oldValue of nil (i.e.
expected to be unset).
* add missing license
* tweak documentation
* document KVSetWithExpiryJSON minimum version