Fixing race condition in AsyncReaderImpl::shutdown

[Myllyenko]

This PR fixes a race condition that occurs upon invocation of AsyncReaderImpl::shutdown when the handlerExecutor field is set.

The problem we faced:

1. User code calls AsyncReaderImpl::shutdown.
2. Then following call chain follows: GrpcStreamRetrier::shutdownImpl -> AsyncReaderImpl::onShutdown -> AsyncReaderImpl::handleReaderClosed.
3. handleReaderClosed sends an action calling eventHandler.onReaderClosed to handlerExecutor but does not wait for execution.
4. AsyncReaderImpl::shutdown returns.
5. User code shutdowns handlerExecutor.
6. handlerExecutor rejects the action from the third step and eventHandler.onReaderClosed is never called.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 46.37%. Comparing base (16f2600) to head (e741432).

Files	Patch %	Lines
...java/tech/ydb/topic/read/impl/AsyncReaderImpl.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #304      +/-   ##
============================================
- Coverage     46.38%   46.37%   -0.02%     
  Complexity     1677     1677              
============================================
  Files           303      303              
  Lines         12109    12108       -1     
  Branches       1208     1208              
============================================
- Hits           5617     5615       -2     
- Misses         6054     6055       +1     
  Partials        438      438              

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