Use background context for renewing dynamic cert in background (#248)

The context available to `renewDynamicCertificate` comes from inside the TLS handshake, and as such
may be bounded by the lifespan of the connection. Passing this into a goroutine will lead to problems
when the connection ends (and the connection context gets canceled with it) but the goroutine is going
to do more I/O on that context.

handshake.go

@@ -727,7 +727,7 @@ func (cfg *Config) renewDynamicCertificate(ctx context.Context, hello *tls.Clien
 
 	// if the certificate hasn't expired, we can serve what we have and renew in the background
 	if timeLeft > 0 {
-		ctx, cancel := context.WithTimeout(ctx, 5*time.Minute)
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
 		go renewAndReload(ctx, cancel)
 		return currentCert, nil
 	}