BUG/MEDIUM: h2: also arm the h2 timeout when sending

BUG/MEDIUM: h2: also arm the h2 timeout when sending

Right now the h2 idle timeout is only set when there is no stream. If we
fail to send because the socket buffers are full (generally indicating
the client has left), we also need to arm it so that we can properly
expire such connections, otherwise some failed transfers might leave
H2 connections pending forever.

Thanks to Thierry Fournier for the diag and the traces.

This patch needs to be backported to 1.8.