bugFix/fix is available timeout

[xieshouyu]

What is the purpose of the change

When the zookeeper is disconnected, the dubbo actor check registry health will be stuck instead of returning to Down
I hope the result is to return to the Down state

The reason is that the isAvailable method calls the getServices method of zookeeper, causing continuous retries. When determining whether it is available, it is necessary to first determine whether it is connected before obtaining it.

Brief changelog

1、Fix the issue of timeout for all calls to the isAvailable method after the zookeeper is disconnected
2、Move the isAvailable of NopServiceDiscovery into its own implementation class

Verifying this change

Start the service, use pfctl or iptables to disconnect zookeeper, and it can return to the down state normally.

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

0.0% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #12796 (3893980) into 3.2 (879fd81) will decrease coverage by 0.50%.
Report is 2 commits behind head on 3.2.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #12796      +/-   ##
============================================
- Coverage     69.95%   69.46%   -0.50%     
+ Complexity      339        2     -337     
============================================
  Files          1871     1648     -223     
  Lines         98847    68265   -30582     
  Branches      18013     9979    -8034     
============================================
- Hits          69152    47422   -21730     
+ Misses        24171    16284    -7887     
+ Partials       5524     4559     -965     

see 3519 files with indirect coverage changes

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