fix: when the routing policy is not set, the setInvokers() method has a co process leak

[caochengxiang]

1、1.5.7版本不指定路由策略下会发生协程泄漏，通过pprof发现下面两处会有协程泄漏，cluster/router/chain/chain.go；
`func (c *RouterChain) AddRouters(routers []router.PriorityRouter) {
newRouters := make([]router.PriorityRouter, 0, len(c.builtinRouters)+len(routers))
newRouters = append(newRouters, c.builtinRouters...)
newRouters = append(newRouters, routers...)
sortRouter(newRouters)
c.mutex.Lock()
defer c.mutex.Unlock()
c.routers = newRouters

go func() {
	c.notify <- struct{}{}
}()

}

// SetInvokers receives updated invokers from registry center. If the times of notification exceeds countThreshold and
// time interval exceeds timeThreshold since last cache update, then notify to update the cache.
func (c *RouterChain) SetInvokers(invokers []protocol.Invoker) {
c.mutex.Lock()
c.invokers = invokers
c.mutex.Unlock()

go func() {
	c.notify <- struct{}{}
}()

}`
这里面两处的c.notify <- struct{}{}会阻塞，因为loop方法不会运行

[codecov-commenter]

Codecov Report

Merging #2104 (adbcda0) into 1.5 (2cb57ab) will decrease coverage by 0.92%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##              1.5    #2104      +/-   ##
==========================================
- Coverage   55.82%   54.90%   -0.93%     
==========================================
  Files         279      267      -12     
  Lines       16071    15151     -920     
==========================================
- Hits         8972     8318     -654     
+ Misses       6154     5952     -202     
+ Partials      945      881      -64     

Impacted Files	Coverage Δ
cluster/cluster_impl/broadcast_cluster_invoker.go	80.00% <ø> (ø)
cluster/cluster_impl/failsafe_cluster_invoker.go	81.81% <ø> (ø)
cluster/cluster_impl/forking_cluster_invoker.go	58.33% <0.00%> (ø)
cluster/loadbalance/consistent_hash.go	90.32% <ø> (ø)
cluster/router/condition/listenable_router.go	12.12% <ø> (ø)
cluster/router/condition/router.go	80.10% <ø> (ø)
cluster/router/tag/file.go	81.81% <ø> (ø)
cluster/router/tag/tag_router.go	44.57% <ø> (ø)
common/extension/service_discovery.go	0.00% <ø> (ø)
common/host_util.go	60.00% <ø> (ø)
... and 88 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[AlexStocks]

pls do not merge this pr until @caochengxiang add an ut.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information