release the allocated memory of the ‘url’ field when destroy invoker

[No-SilverBullet]

What would you like to be added:

dubbo-go/protocol/invoker.go

Lines 92 to 96 in faf6f2a

	func (bi *BaseInvoker) Destroy() {
	logger.Infof("Destroy invoker: %s", bi.GetURL())
	bi.destroyed.Store(true)
	bi.available.Store(false)
	}

here is my question, since the invoker is destroyed ,so why do we not set the url field in invoker to nil ,like below

func (bi *BaseInvoker) Destroy() {
logger.Infof("Destroy invoker: %s", bi.GetURL())
bi.url = nil //add this
bi.destroyed.Store(true)
bi.available.Store(false)
}

Why is this needed:
in this line, the invoker will be add to a global var dubboProtocol

dubbo-go/protocol/dubbo/dubbo_protocol.go

Line 98 in faf6f2a

dp.SetInvokers(invoker)

dubbo-go/protocol/dubbo/dubbo_protocol.go

Line 59 in faf6f2a

var dubboProtocol *DubboProtocol

so when destroy the invoker, the 'url' is useless and still it cannot be gc（unless destroy the global var）,which i think may cause memory leaks

[mark4z]

#2223