Add Config.GetConcurrency with default fallback

internal/cli/run_proxy.go

 		EventStream:     eventStream,
 
 		Secret:                      conf.Secret,
+		Concurrency:                 conf.GetConcurrency(mtglib.DefaultConcurrency),
 		DomainFrontingPort:          conf.GetDomainFrontingPort(mtglib.DefaultDomainFrontingPort),
 		DomainFrontingIP:            conf.GetDomainFrontingIP(nil),
 		DomainFrontingProxyProtocol: conf.GetDomainFrontingProxyProtocol(false),

internal/config/config.go

 	} `json:"stats"`
 }
 
+func (c *Config) GetConcurrency(defaultValue uint) uint {
+	if concurrency := c.Concurrency.Get(0); concurrency != 0 {
+		return concurrency
+	}
+	return c.Concurrency.Get(defaultValue)
+}
+
 func (c *Config) GetDNS() *url.URL {
 	var dohURL *url.URL