Fix unknown exception caused by thread safety issue

[315157973]

Motivation

Both the response thread and timeout detection thread will access PendingProduceCallback
This problem will easily occur when the sending latency is close to the timeout threshold.

Step-1： response thread and timeout detection thread access 1 at the same time, none of them meet the conditions for return

Step-2: timeout detection thread execute to 2, and set responseMap = null
Setp-3: response thread execute to 3, cause NPE

Modifications

Add synchronized

Documentation

• no-need-doc

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The spotbugs check failed:

Medium: Inconsistent synchronization of io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback.responseMap; locked 60% of time [io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback, io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback, io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback, io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback, io.streamnative.pulsar.handlers.kop.storage.ReplicaManager$PendingProduceCallback] Unsynchronized access at ReplicaManager.java:[line 119]Unsynchronized access at ReplicaManager.java:[line 121]Synchronized access at ReplicaManager.java:[line 127]Synchronized access at ReplicaManager.java:[line 125]Synchronized access at ReplicaManager.java:[line 133] IS2_INCONSISTENT_SYNC

You need to check the synchronized code blocks again.

BTW, in our private fork, this pending produce is removed and the timeout field is not processed.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (aa2f171) 16.98% compared to head (69ef235) 16.96%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2033      +/-   ##
============================================
- Coverage     16.98%   16.96%   -0.03%     
+ Complexity      730      728       -2     
============================================
  Files           191      191              
  Lines         14310    14310              
  Branches       1339     1339              
============================================
- Hits           2430     2427       -3     
- Misses        11704    11705       +1     
- Partials        176      178       +2     

Files	Coverage Δ
...ve/pulsar/handlers/kop/storage/ReplicaManager.java	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes