fix: Issue/806: Add a lock to prevent the disposal of the timer while it is running.

[OrezzerO]

• fix: Issue/806

Closes #806 806

Proposed Changes

Add a lock to prevent the disposal of the timer while it is running.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• mvn test completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.50%. Comparing base (7fe8bdc) to head (ad89f32).

Files with missing lines	Patch %	Lines
...nternal/flowable/FlowableBufferTimedFlushable.java	77.77%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #807      +/-   ##
============================================
- Coverage     88.51%   88.50%   -0.01%     
+ Complexity      787      783       -4     
============================================
  Files           174      174              
  Lines          7207     7214       +7     
  Branches        405      407       +2     
============================================
+ Hits           6379     6385       +6     
  Misses          697      697              
- Partials        131      132       +1     

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[jerdaane]

Hi @OrezzerO
Thank you for the work on this PR! I see that it addresses an issue that impacts my usage of the Client API, and having it merged and released would make my application more reliable.
I was wondering if there are any blockers or additional steps needed before this can be merged and released.
Looking forward to your thoughts!
Best,
Jeroen

[OrezzerO]

This PR has some conflicts, now I solve it. Please take a look at it @bednar ,thank you!