Feat/direct notify

[LaurenceLiZhixin]

What this PR does:
This pr does: when provider is close, it notify consumer through getty-session, not only notified by registry.
This is quicker for consumer to delete target invoker in directory.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:
I add a common sync.Map in common/dubbo_dir_refresh.go, which links dubbo-getty and registryDirectory.
when session is on close or on errror, it call dir.Notify(deleteEvent) to refresh quickly.

Does this PR introduce a user-facing change?:

NONE

[codecov-io]

Codecov Report

Merging #995 (dad63a5) into develop (0cb6edb) will increase coverage by 0.02%.
The diff coverage is 59.09%.

@@             Coverage Diff             @@
##           develop     #995      +/-   ##
===========================================
+ Coverage    59.66%   59.68%   +0.02%     
===========================================
  Files          261      261              
  Lines        12919    12928       +9     
===========================================
+ Hits          7708     7716       +8     
- Misses        4240     4241       +1     
  Partials       971      971              

Impacted Files	Coverage Δ
cluster/router/chain/chain.go	67.96% <0.00%> (ø)
cluster/router/healthcheck/health_check_route.go	73.07% <ø> (+2.70%)	⬆️
cluster/router/tag/tag_router.go	73.84% <ø> (+1.25%)	⬆️
registry/directory/directory.go	77.27% <28.57%> (-1.70%)	⬇️
remoting/getty/pool.go	63.51% <75.00%> (+0.16%)	⬆️
remoting/getty/listener.go	42.48% <77.77%> (+1.40%)	⬆️
remoting/getty/getty_client.go	44.30% <100.00%> (ø)
...tocol/rest/server/server_impl/go_restful_server.go	48.78% <0.00%> (+4.87%)	⬆️

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[zouyx]

Why use public variable in this module? Although you have comment in this variable，but It can be used in all modules in dubbo-go.Even the module out of dubbo-go.

[LaurenceLiZhixin]

I think it's the best way to achieve the direct notify from remoting.session to directory

[LaurenceLiZhixin]

I can't find another way to link this two pkg. As there may be unexpected loop refer.

[zouyx]

请允许我使用中文，关于这个问题，我先说一下我的看法。
先看一下现在的结构，我没理解错应该是：
invoker-》client-》getty pool-》session（connection）
现在是直接加在 session 的 close/error 上。

针对这个 PR，我有以下几个疑问：

• 这个功能是否应该加在 invoker / client ？为什么我这么说？请看第二个问题。
• 因为有可能有 10 个 connection，其中 n （ n < 10 ）个因为网络抖动连不上，然后就去删除 DIr 中的 cache？这也可能和第三个问题有关联。
• 我只看到在 session（connection）上删除，但是我没看到如何加回去？
• 如果按照现在的套路，如果 n ( n > 1) 个 registry 要加回去，是不是会造成：频繁增加。

我的建议是：
调整到 invoekr 或者 client，通过判断窗口期中多少个请求失败，再触发删除 cache，最后记得加上恢复 registry 的代码。
但是如果按照我的建议做的话，是否能基于：ActiveFilter？

[LaurenceLiZhixin]

我仔细想了想，这里的确有问题。
我写这个pr的是以：“这个session一旦被关闭，证明server端就一定关闭了”为前提的。一旦session因为网络抖动出现了err，会直接删除掉dir的cache，这样设计不太合理。
我先表述一下这个pr的初衷：是为了提高客户端对于server端下线的感知速度：不只通过regisgtry感知，而是通过session感知。如果判断窗口期请求，我觉得是不太现实，因为这个时间可能registry都通知过来了。
感觉这个需求可能有点问题。
另一方面，我的另一个pr：#976
有提供invoke前的健康检查机制，感觉这两个pr在功能上是有重复的，我和展图讨论下。