MEDIUM: h2: prepare a graceful shutdown when the frontend is stopped

MEDIUM: h2: prepare a graceful shutdown when the frontend is stopped

During a reload operation, instead of keeping the H2 connections opened
forever causing confusion during configuration changes, let's send a
graceful shutdown so that the client knows that it would better open a
new connection for future requests. We can't really catch the signal
from H2, but we can advertise this graceful shutdown upon the next I/O
event (eg: a WINDOW_UPDATE from the client or a new request). One of
the visible effect is that the old process quits much faster.

This patch should be backported to 1.8 since it is affected by this
problem.