The function clears all stun-handlers. This causes all of those
objects to be destroyed.
A handler can have a pending connecter, which was only killed in
case CloseConnection() was called. This is never the case when
the object is destroyed. In result, the connecter could finish
and cause a use-after-free by calling into the (now deleted)
handler.
TURN is a last resort, used only if all other methods failed.
TURN is a relay approach to connect client and server together, where
openttd.org (by default) is the middleman.
It is very unlikely either the client or server cannot connect to
the STUN server, as they are both already connected to the Game
Coordinator. But in the odd case it does fail, estabilishing the
connection fails without any further possibility to recover.
