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 :warning:

Additional details and impacted files

```diff @@ 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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