fix code bug & optimize the memory usage in DefaultMappedFile

[SeaItFover]

Make sure set the target branch to develop

What is the purpose of the change MappedFileQueue: if the initial cursor of listIterator equals zero,the previous element will always null. MappedFile: use AtomicIntegerFieldUpdater instead of AtomicInteger.

Brief changelog

Verifying this change DefaultMappedFile class

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[Oliverwqcwrw]

Hello @SeaItFover

Thank you very much for taking the time to contribute to the community, your observation is very keen, I noticed that CI did not pass, please check it first, by the way, you can first create an issue to describe the problem, and then your PR can associate this issue :)

[SeaItFover]

Hello @SeaItFover

Thank you very much for taking the time to contribute to the community, your observation is very keen, I noticed that CI did not pass, please check it first, by the way, you can first create an issue to describe the problem, and then your PR can associate this issue :)

emmm...我改动很小，不知道哪里有问题

[Oliverwqcwrw]

You can click Details to see the Details of the error

[SeaItFover]

Thanks very much. Hope that my first pr can be merged.

[coveralls]

Coverage increased (+0.07%) to 48.801% when pulling 3f242e8aa17f3a71906c791f628bb0a2747e6fd3 on SeaItFover:develop into 8504abfafcf763be6fc593183edb1a2f8f061e51 on apache:develop.

[codecov-commenter]

Codecov Report

Merging #4678 (3f242e8) into develop (8504abf) will decrease coverage by 0.11%. The diff coverage is 47.05%.

@@              Coverage Diff              @@
##             develop    #4678      +/-   ##
=============================================
- Coverage      44.68%   44.57%   -0.12%     
+ Complexity      7540     7519      -21     
=============================================
  Files            977      977              
  Lines          67594    67595       +1     
  Branches        8927     8927              
=============================================
- Hits           30207    30128      -79     
- Misses         33669    33742      +73     
- Partials        3718     3725       +7     

Impacted Files	Coverage Δ
...ava/org/apache/rocketmq/store/MappedFileQueue.java	63.77% <0.00%> (ø)
...ache/rocketmq/store/logfile/DefaultMappedFile.java	48.40% <48.48%> (+0.16%)	:arrow_up:
...rg/apache/rocketmq/common/stats/StatsSnapshot.java	84.61% <0.00%> (-15.39%)	:arrow_down:
...apache/rocketmq/broker/longpolling/PopRequest.java	31.03% <0.00%> (-13.80%)	:arrow_down:
...org/apache/rocketmq/common/stats/StatsItemSet.java	41.79% <0.00%> (-13.44%)	:arrow_down:
.../apache/rocketmq/common/stats/MomentStatsItem.java	38.09% <0.00%> (-9.53%)	:arrow_down:
...ache/rocketmq/common/stats/MomentStatsItemSet.java	39.13% <0.00%> (-8.70%)	:arrow_down:
...va/org/apache/rocketmq/common/stats/StatsItem.java	50.00% <0.00%> (-5.00%)	:arrow_down:
...ketmq/common/protocol/body/ConsumerConnection.java	95.83% <0.00%> (-4.17%)	:arrow_down:
...he/rocketmq/controller/impl/DLedgerController.java	70.65% <0.00%> (-2.72%)	:arrow_down:
... and 14 more

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[SeaItFover]

I see that all checks have been approved, when to be merged.

[Oliverwqcwrw]

Hello, @SeaItFover First of all, we can't timely merger your pr, because all the PMC and Commiters has his own work, everyone is using their free time to contribute to the community, so the review of pr is not certain, as you can see, there are a lot of pr is waiting for review, so I hope you can wait patiently, The community very welcomes contributions from developers