This allows us to change the max chunk length on a per chunk basis
instead of for the whole stream. It's also much easier to use this
way even if we don't want to change the chunk size.
Defaults to a maximum of 1000000 bytes.
Also standardize the te_identity and te_chunked decoding functions.
Now they both try to read as much as possible (up to the limit),
making body reading much faster when not using chunked encoding.
