Fixing race condition between start() and close() in TopicRetryableStream

[Myllyenko]

start() checked isClosed before creating the stream and only published it afterwards. When close() ran in between it found an empty realStream slot, returned false - which WriterImpl reads as "the stream will never call onClose", so it finishes the shutdown - and start() then went on to open a stream that nothing would ever close. It stayed connected until the server gave up on it.

Re-check isClosed once the stream is published and started, and close it if the slot is still ours. The check has to come after start() because closing a stream that was never started would half-close a gRPC call that has not begun.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.43%. Comparing base (d876581) to head (5add468).
⚠️ Report is 44 commits behind head on master.

Files with missing lines	Patch %	Lines
...java/tech/ydb/topic/impl/TopicRetryableStream.java	66.66%	0 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (66.66%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #707      +/-   ##
============================================
+ Coverage     71.91%   72.43%   +0.52%     
- Complexity     3474     3524      +50     
============================================
  Files           390      391       +1     
  Lines         16220    16344     +124     
  Branches       1698     1703       +5     
============================================
+ Hits          11664    11839     +175     
+ Misses         3904     3866      -38     
+ Partials        652      639      -13     

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