BUG/MEDIUM: http: wait for the exact amount of body bytes in wait_for_request_body

BUG/MEDIUM: http: wait for the exact amount of body bytes in wait_for_request_body

Due to the fact that we were still considering only msg->sov for the
first byte of data after calling http_parse_chunk_size(), we used to
miscompute the input data size and to count the CRLF and the chunk size
as part of the input data. The effect is that it was possible to release
the processing with 3 or 4 missing bytes, especially if they're typed by
hand during debugging sessions. This can cause the stats page to return
some errors in admin mode, and the url_param balance algorithm to fail
to properly hash a body input.

This fix must be backported to 1.5.