Ftr: start provider with random port

[zouyx]

What this PR does:

• if want to use random port in provider then don't provide port in server config file , like

protocols:
  "dubbo":
    name: "dubbo"
#    port: 20000

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #476 into develop will increase coverage by 0.12%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           develop     #476      +/-   ##
===========================================
+ Coverage    66.39%   66.51%   +0.12%     
===========================================
  Files          184      184              
  Lines         9700     9718      +18     
===========================================
+ Hits          6440     6464      +24     
+ Misses        2624     2616       -8     
- Partials       636      638       +2     

Impacted Files	Coverage Δ
config/service_config.go	71.31% <75.00%> (+0.14%)	⬆️
cluster/cluster_impl/failback_cluster_invoker.go	78.49% <0.00%> (-2.16%)	⬇️
remoting/kubernetes/listener.go	56.07% <0.00%> (ø)
protocol/dubbo/client.go	69.09% <0.00%> (+1.21%)	⬆️
protocol/dubbo/readwriter.go	70.73% <0.00%> (+2.43%)	⬆️
protocol/dubbo/codec.go	77.50% <0.00%> (+5.00%)	⬆️
cluster/cluster_impl/base_cluster_invoker.go	72.46% <0.00%> (+10.14%)	⬆️

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[AlexStocks]

there are three things left to do. Number 1, wait for the latest dubbogo/getty pr. Number 2, @zouyx pls change your example in ur first comment to let followers know how to use this feature clearly.

for examples:

# server.yml
protocols:
  "dubbo":
    name: "dubbo"
#    port: 20000

or

# server.yml
protocols:
  "dubbo":
    name: "dubbo"
    ip: "192.168.0.1"

or other examples.

Number 3, if the server or the server examples of dubbo-samples do not support set ip, pls add or improve it if you think the binding-ip is a important feature.

[zouyx]

I think this is the best implement in random port , because it compatibility with jsonrpc ,grpc and rest, that three protocols are not use getty to start server .

rest:

dubbo-go/protocol/rest/server/server_impl/go_restful_server.go

Line 76 in f1de848

err := grs.srv.Serve(ln)

jsonrpc:

dubbo-go/protocol/jsonrpc/server.go

Line 232 in f1de848

func (s *Server) Start(url common.URL) {

grpc:

dubbo-go/protocol/grpc/server.go

Line 103 in f1de848

if err = server.Serve(lis); err != nil {

[fangyincheng]

LGTM

[hxmhlt]

LGTM

[AlexStocks]

LGTM