Fix the potential access to upsert metadata manager after it is closed

[Jackie-Jiang]

Fix the race condition which can cause access to the upsert metadata after the manager is closed:

• Thread 1 finishes the _stopped flag check but not invoking startOperation() yet (it might block on snapshot lock)
• Thread 2 stops and closes the metadata manager
• Thread 1 calls startOperation() and start accessing the metadata

In this PR, make startOperation() return false if the pending operations (reference count) already drops to 0, and also merge stopped flag check into startOperation() to simplify the logic. This is not super performance sensitive, so using very tight synchronized block should be fine

[codecov-commenter]

Codecov Report

Merging #11692 (80fb565) into master (1748be4) will increase coverage by 0.02%.
Report is 5 commits behind head on master.
The diff coverage is 42.55%.

@@             Coverage Diff              @@
##             master   #11692      +/-   ##
============================================
+ Coverage     63.02%   63.04%   +0.02%     
  Complexity     1120     1120              
============================================
  Files          2343     2343              
  Lines        125672   125672              
  Branches      19314    19317       +3     
============================================
+ Hits          79209    79236      +27     
+ Misses        40808    40782      -26     
+ Partials       5655     5654       -1     

Flag	Coverage Δ
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (ø)
integration2	0.00% <0.00%> (ø)
java-11	63.03% <42.55%> (+12.97%)	⬆️
java-17	62.90% <42.55%> (+48.45%)	⬆️
java-20	14.43% <0.00%> (-48.46%)	⬇️
temurin	63.04% <42.55%> (+0.02%)	⬆️
unittests	63.04% <42.55%> (+0.02%)	⬆️
unittests1	67.19% <42.55%> (+<0.01%)	⬆️
unittests2	14.45% <0.00%> (+0.01%)	⬆️

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Files	Coverage Δ
...cal/upsert/BasePartitionUpsertMetadataManager.java	40.61% <42.55%> (+1.00%)	⬆️

... and 14 files with indirect coverage changes

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