MINOR: h1: add the restart offsets into struct h1m

MINOR: h1: add the restart offsets into struct h1m

Currently the only user of struct h1m is the h2 mux when it has to parse
an H1 message coming from the channel. Unfortunately this is not enough
to efficiently parse HTTP/1 messages like those coming from the network
as we don't want to restart from scratch at every byte received.

This patch reintroduces the "next" offset into the H1 message so that any
H1 parser can use it to restart when called with a state that is not the
initial state.