A connection refused before auth used to keep its place among
max_unauthenticated while it lingered (up to 1 s), so refusals could
keep real clients out. Now `refuse` gives that place back and takes one
of a separate, smaller set of lingering places (Limits::max_lingering,
8) before the reply is written. When none is free the connection is
closed right after the FIN instead of lingering, so threads stay
bounded as #1806 intended.
Tests: a_lingering_connection_holds_no_place_to_authenticate and
only_so_many_refused_connections_linger.
skylar taylor-barrick
bb0ed18133