MAJOR: http/compression: fix chunked-encoded response processing

MAJOR: http/compression: fix chunked-encoded response processing

Now we have valid buffer offsets, we can use them to safely parse the
input and only forward when needed. Thus we can get rid of the
consumed_data accumulator, and the code now works both for chunked and
content-length, even with a server feeding one byte at a time (which
systematically broke the previous one).

It's worth noting that 0<CRLF> must always be sent after end of data
(ie: chunk_len==0), and that the trailing CRLF is sent only content
length mode, because in chunked we'll have to pass trailers.