mod_c2s,mod_s2s: Fix error on shutdown (Thanks Martin)

Since there are two calls to done() that can happen, if the timing is
right (or wrong) both can happen, which previously triggered an error.

plugins/mod_c2s.lua

@@ -433,7 +433,7 @@ module:hook("server-stopping", function(event) -- luacheck: ignore 212/event
 end, -80);
 
 module:hook("server-stopping", function(event)
-	local wait, done = async.waiter();
+	local wait, done = async.waiter(1, true);
 	module:hook("c2s-closed", function ()
 		if next(sessions) == nil then done(); end
 	end)

plugins/mod_s2s.lua

@@ -973,7 +973,7 @@ module:hook("server-stopping", function(event)
 		end
 	end
 
-	local wait, done = async.waiter();
+	local wait, done = async.waiter(1, true);
 	module:hook("s2s-closed", function ()
 		if next(sessions) == nil then done(); end
 	end, 1)