MINOR: mux-h2: permit to fix a minimum value for the advertised streams limit

MINOR: mux-h2: permit to fix a minimum value for the advertised streams limit

When using rq-load on tune.h2.fe.max-concurrent-streams, it's easy to
reach a situation where only one stream is allowed. There's nothing
wrong with this but it turns out that slightly higher values do not
necessarily cause significantly higher loads and will improve the user
experience. For this reason the keyword now also supports "min" to
specify a value. Experimentation shows that values from 5 to 15 remain
very effective at protecting the run queue while allowing a great level
of parallelism that keeps a site fluid.