BUG/MEDIUM: h1: ensure the chunk size parser can deal with full buffers

BUG/MEDIUM: h1: ensure the chunk size parser can deal with full buffers

The HTTP/1 code always has the reserve left available so the buffer is
never full there. But with HTTP/2 we have to deal with full buffers,
and it happens that the chunk size parser cannot tell the difference
between a full buffer and an empty one since it compares the start and
the stop pointer.

Let's change this to instead deal with the number of bytes left to process.

As a side effect, this code ends up being about 10% faster than the previous
one, even on HTTP/1.