stream-clone: bail-out earlier if destination repo is not empty The `canperformstreamclone` function is bit messy. However it seems clearer to me to process the very generic condition about "can we consider a stream-clone at all", before checking if a stream-clone is requested and if a compatible protocol can be negotiated with the server. So I am doing some gratuitous movement so reorder conditional.

stream-clone: check the version of streaming clone supported by the client Make the server refuse to produce streaming clone bundle, if the client doesn't specify the stream=v2 capability. This is in preparation to introduce stream=v3. As far as I can tell, this capability was added at the same time as support for bundle2-based streaming pulls was added, so I don't expect clients to break. (the modern client doesn't break, at any rate)

stream-clone: make sure the `stream` capability is set when bundling This is important to start narrowing protocol option in the next changesets.

stream-clone: upgrade the error message for bad stream request The new version if more compact and more consistent with the general Mercurial usage.

stream-clone: yield cache entry in `_entries_walk` too The new function now cover the same ground as _v2_walk.

stream-clone: introduce a _entries_walk That function insert itself between the store and `_v2_walk`. It only deals with StoreEntry unlike `_v2_walk` that deal with actual file. A share of the `_v2_walk` logic will be moved in this new `_entry_walk` function. Having this function will help us to implement a new "v3" version of the protocol that will be more entry centric.

store: make `walk` return an entry for obsolescence if requested so Instead of having dedicated code in the streamclone code, we should have the store deal with advertising the data it contains.