MINOR: mux-h2: permit to moderate the advertised streams limit depending on load

MINOR: mux-h2: permit to moderate the advertised streams limit depending on load

Global setting tune.h2.fe.max-concurrent-streams now supports an optional
"rq-load" option to pass either a target load, or a keyword among "auto"
and "ignore". These are used to quadratically reduce the advertised streams
limit when the thread's run queue size goes beyong the configured value,
and automatically reduce the load on the process from new connections.
With "auto", instead of taking an explicit value, it uses as a target the
"tune.runqueue-depth" setting (which might be automatic). Tests have shown
that values between 50 and 100 are already very effective at reducing the
loads during attacks from 100000 to around 1500. By default, "ignore"
is in effect, which means that the dynamic tuning is not enabled.