fix aggressive expiration detection (#11446)

for some flaky networks this may be too fast of a value
choose a defensive value, and let this be addressed
properly in a new refactor of dsync with renewal logic.

Also enable faster fallback delay to cater for misconfigured
IPv6 servers

refer
 - https://golang.org/pkg/net/#Dialer
 - https://tools.ietf.org/html/rfc6555

cmd/http/dial_dnscache.go

@@ -44,7 +44,9 @@ func DialContextWithDNSCache(cache *DNSCache, baseDialCtx DialContext) DialConte
 		baseDialCtx = (&net.Dialer{
 			Timeout:   30 * time.Second,
 			KeepAlive: 30 * time.Second,
-			DualStack: true,
+			// If zero, Go defaults to '300ms', we will default to 100ms instead.
+			// https://tools.ietf.org/html/rfc6555
+			FallbackDelay: 100 * time.Millisecond,
 		}).DialContext
 	}
 	return func(ctx context.Context, network, host string) (net.Conn, error) {