Avoid scheduler thread blocking

[alex268]

Fixed #262

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 43.96%. Comparing base (273c93f) to head (645c8f9).

Files	Patch %	Lines
.../java/tech/ydb/core/impl/pool/GrpcChannelPool.java	81.25%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #265      +/-   ##
============================================
- Coverage     43.99%   43.96%   -0.03%     
- Complexity     1550     1554       +4     
============================================
  Files           294      294              
  Lines         11359    11359              
  Branches       1109     1112       +3     
============================================
- Hits           4997     4994       -3     
- Misses         5966     5967       +1     
- Partials        396      398       +2     

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