bundle2: support chunk iterator as part data
When the `part.data` attribute is an iterator, we assume it is an iterator of
chunks and use it.
We use a chunkbuffer to yield chunks of 4096 bytes.
The tests are updated to use this feature.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.