perf: replace concatMap to flatMapSequential to improve parallelism and efficiency

[guqing]

What type of PR is this?

/kind improvement
/area core
/milestone 2.20.x

What this PR does / why we need it:

将 concatMap 替换为 flatMapSequential 以提高并行度和执行效率

可以看一下这个场景示例来模拟像文章列表 API 的数据组装
假如每个步骤的执行时间是 1s 有 4 个步骤 同时 Flux 发出 4 条数据:

@Test  
void test() {  
    var startMs = System.currentTimeMillis();  
  
    var monoA = Mono.fromSupplier(  
            () -> {  
                sleep();  
                return "A";  
            })        .subscribeOn(Schedulers.boundedElastic());  
  
    var monoB = Mono.fromSupplier(  
            () -> {  
                sleep();  
                return "B";  
            })        .subscribeOn(Schedulers.boundedElastic());  
  
    var monoC = Mono.fromSupplier(  
            () -> {  
                sleep();  
                return "C";  
            })        .subscribeOn(Schedulers.boundedElastic());  
  
    var monoD = Mono.fromSupplier(  
            () -> {  
                sleep();  
                return "D";  
            })        .subscribeOn(Schedulers.boundedElastic());  
  
    var convert = Mono.when(monoA, monoB, monoC, monoD);  

    Flux.just("1", "2", "3", "4")
        // concatMap(convert::thenReturn)
        .flatMapSequential(convert::thenReturn)  
        .collectList()  
        .block(); 

    System.out.println("Time: " + (System.currentTimeMillis() - startMs));  
}

private static void sleep() {  
    try {  
        Thread.sleep(1000);  
    } catch (InterruptedException e) {  
        throw new RuntimeException(e);  
    }
}

结果:

1. 如果每个步骤没有加 subscribeOn 且使用 concatMap 耗时: 16362 ms
2. 每个步骤使用 subscribeOn 且使用 concatMap 耗时: 4174 ms
3. 每个步骤使用 subscribeOn 且使用 flatMapSequential 耗时: 1185 ms

Does this PR introduce a user-facing change?

提升页面访问速度

[codecov]

Codecov Report

Attention: Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.

Project coverage is 56.99%. Comparing base (c945a94) to head (e1d6c1c).
Report is 96 commits behind head on main.

Files with missing lines	Patch %	Lines
...ava/run/halo/app/content/impl/PostServiceImpl.java	0.00%	2 Missing ⚠️
...in/java/run/halo/app/plugin/PluginServiceImpl.java	0.00%	2 Missing ⚠️
...run/halo/app/content/comment/ReplyServiceImpl.java	0.00%	1 Missing ⚠️
...n/halo/app/content/impl/SinglePageServiceImpl.java	0.00%	1 Missing ⚠️
...me/finders/impl/CommentPublicQueryServiceImpl.java	66.66%	1 Missing ⚠️
.../app/theme/finders/impl/ContributorFinderImpl.java	0.00%	1 Missing ⚠️
...un/halo/app/theme/finders/impl/PostFinderImpl.java	0.00%	1 Missing ⚠️
.../finders/impl/SinglePageConversionServiceImpl.java	0.00%	1 Missing ⚠️
...run/halo/app/theme/finders/impl/TagFinderImpl.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6706      +/-   ##
============================================
- Coverage     58.17%   56.99%   -1.18%     
- Complexity     3947     3978      +31     
============================================
  Files           680      713      +33     
  Lines         23262    24009     +747     
  Branches       1582     1572      -10     
============================================
+ Hits          13532    13685     +153     
- Misses         9106     9712     +606     
+ Partials        624      612      -12     

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[JohnNiang]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud