client: add client-side caching
client: add client-side caching

Ideally we should cache the subtrees as well, but it is not easy
to expose this with the current API.

Signed-off-by: Thomas Gazagnaire <5f50a84c1fa3bcff146405017f36aec1a10a9e38@gazagnaire.org>

docker/datakit,docker/datakit,docker/datakit