fix(discovery):Fix the protected data race

[pretty66]

After the program starts, one of the interfaces is requested, and the warning level data contention will be triggered after 60 seconds

func (d *Discovery) exitProtect() {
    // exist protect mode after two renew cycle， data race after 60s
    time.Sleep(time.Second * 60)
    d.protected = false
}

INFO 12/25-11:51:59.456 http-access-log stack=<nil> timeout_quota=39.999948634 ip=127.0.0.1 user=no_user params= msg=0 ts=0.000798093 traceid= ret=0 method=GET path=/discovery/fetch/all err= 
==================
WARNING: DATA RACE
Write at 0x00c00018c8a8 by goroutine 26:
  github.com/bilibili/discovery/discovery.(*Discovery).exitProtect()
      /Users/work/discovery/discovery/discovery.go:41 +0x51

Previous read at 0x00c00018c8a8 by goroutine 33:
  github.com/bilibili/discovery/discovery.(*Discovery).Protected()
      /Users/work/discovery/discovery/syncup.go:24 +0x59
  github.com/bilibili/discovery/http.initProtect()
      /Users/work/discovery/http/http.go:50 +0x2f
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Context).Next()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/context.go:84 +0xa6
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Logger.func1()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/logger.go:26 +0x174
  github.com/go-kratos/kratos/pkg/net/http/blademaster.HandlerFunc.ServeHTTP()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:72 +0x3d
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Handler.ServeHTTP-fm()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:64 +0x50
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Context).Next()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/context.go:84 +0xa6
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Trace.func1()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/trace.go:38 +0x8a3
  github.com/go-kratos/kratos/pkg/net/http/blademaster.HandlerFunc.ServeHTTP()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:72 +0x3d
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Handler.ServeHTTP-fm()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:64 +0x50
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Context).Next()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/context.go:84 +0xa6
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Recovery.func1()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/recovery.go:30 +0x6c
  github.com/go-kratos/kratos/pkg/net/http/blademaster.HandlerFunc.ServeHTTP()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:72 +0x3d
  github.com/go-kratos/kratos/pkg/net/http/blademaster.Handler.ServeHTTP-fm()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:64 +0x50
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Context).Next()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/context.go:84 +0xa6
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Engine).handleContext()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:327 +0x5f8
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Engine).ServeHTTP()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:490 +0x252
  net/http.serverHandler.ServeHTTP()
      /usr/local/Cellar/go/1.14.5/libexec/src/net/http/server.go:2836 +0xce
  net/http.(*conn).serve()
      /usr/local/Cellar/go/1.14.5/libexec/src/net/http/server.go:1924 +0x837

Goroutine 26 (running) created at:
  github.com/bilibili/discovery/discovery.New()
      /Users/work/discovery/discovery/discovery.go:34 +0x21d
  main.main()
      /Users/work/discovery/cmd/discovery/main.go:23 +0x14d

Goroutine 33 (running) created at:
  net/http.(*Server).Serve()
      /usr/local/Cellar/go/1.14.5/libexec/src/net/http/server.go:2962 +0x5b6
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Engine).RunServer()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:462 +0xc1
  github.com/go-kratos/kratos/pkg/net/http/blademaster.(*Engine).Start.func1()
      /Users/work/gopath/pkg/mod/github.com/go-kratos/kratos@v0.6.0/pkg/net/http/blademaster/server.go:103 +0x77
==================

[tallongsun]

just curious about shouldn't exiting protected mode after all instances finish register?