No way to get important metadata

[fasaxc]

One issue that we found in writing our product using etcd3gw is that the methods don't return the metadata from the response "envelope". There are various cases where that information is needed:

• To do watches correctly, you need the response revision, not the per-key revision. This is because the per-key revision may be pre-compaction and you can't watch on a compacted revision.
• To do chunked gets (with limit=<n>) correctly, you need the "more" flag form the response and the response revision.

Perhaps you could switch to a dedicated Response object that exposes the KVs and other metadata?

[dims]

@fasaxc the main consumer of this library is tooz. as long as we don't break tooz, we should be fine with making changes. would you be able to help with the changes?