We generally want a mechanism to pass information about the
generated bundle back to callers (in addition to the byte stream).
Ideally we would return a bundler from this function and have the
caller code to an interface. But the bundling APIs are not great
and getbundlechunks() is the best API we have for obtaining bundle
contents in a unified manner.
We change getbundlechunks() to return a dict that we can use to
communicate metadata.
We populate that dict with the bundle version number to demonstrate
some value.
.. api::
exchange.getbundlechunks() now returns a 2-tuple instead of just an iterator.