Memory tests

We need some regression tests that verify that memory use is not excessive. A test structure that can measure peak memory usage perhaps and assert it remaining below a given threshold.

This is a non-exhaustive list of tests that need implementing:
Relating to HTTP parsing:

1. Test very long request line
2. Test very long response line
3. Test very long header line
4. Test many headers
5. Test chunked long size line
6. Test chunked long trailer line
7. Test chunked many trailers
8. Test BodyPartReaderPayload.write() with a large compressed payload

For multipart.py we need a similar list of tests specific to multipart requests.

Likely other tests that are useful elsewhere too.

On a side note, I wonder if the (buffer) limit always be a minimum of double the line limit, to ensure we don't pause the socket reading too much?