BUG/MEDIUM: h1: Make sure we subscribe before going into idle list.

BUG/MEDIUM: h1: Make sure we subscribe before going into idle list.

In h1_detach(), make sure we subscribe before we call
srv_add_to_idle_list(), not after. As soon as srv_add_to_idle_list() is
called, and it is put in an idle list, another thread can take it, and
we're no longer allowed to subscribe.
This fixes a race condition when another thread grabs a connection as soon
as it is put, the original owner would subscribe, and thus the new thread
would fail to do so, and to activate polling.