BUG/MEDIUM: dns: Reset reconnect tempo when connection is finally established

The issue was introduced by commit 27236f221 ("BUG/MINOR: dns: add tempo
between 2 connection attempts for dns servers"). In this patch, to delay the
reconnection, a timer is used on the appctx when it is created. This
postpones the appctx initialization. However, once initialized, the
expiration time of the underlying task is not reset. So, it is always
considered as expired and the appctx is woken up in loop.

The fix is quite simple. In dns_session_init(), the expiration time of the
appctx's task is alwaus set to TICK_ETERNITY.

This patch must be backported everywhere the commit above was backported. So
as far as 2.8 for now but possibly to all stable versions.

src/dns.c

@@ -832,6 +832,7 @@ static int dns_session_init(struct appctx *appctx)
 
 	applet_expect_no_data(appctx);
 	ds->appctx = appctx;
+	appctx->t->expire = TICK_ETERNITY;
 	return 0;
 
   error: