Fix unimportant deadlock in TBlockDevice and add new UT

[va-kuznecov]

Changelog entry

Deadlock can occur only if BufferPoolSize is 4, so maxEnqueuedCompletionActions is 2. This is almost impossible configuration of PDisk, so it's not important in real life

Changelog category

• Bugfix

Additional information

...

[github-actions]

⚪ 2024-10-02 15:48:05 UTC Pre-commit check linux-x86_64-relwithdebinfo for 0830c29 has started.
⚪ 2024-10-02 15:48:08 UTC Artifacts will be uploaded here
⚪ 2024-10-02 15:50:59 UTC ya make is running...
🟡 2024-10-02 17:08:19 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
15755	14268	0	3	1377	107

⚪ 2024-10-02 17:09:39 UTC ya make is running... (failed tests rerun, try 2)
🟢 2024-10-02 17:21:14 UTC Tests successful.

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
108 (only retried tests)	15	0	0	0	93

🟢 2024-10-02 17:21:23 UTC Build successful.
🟡 2024-10-02 17:21:48 UTC ydbd size 2.8 GiB changed* by +208.2 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 8030e8b	merge: 0830c29	diff	diff %
ydbd size	3 001 455 544 Bytes	3 001 668 728 Bytes	+208.2 KiB	+0.007%
ydbd stripped size	477 330 168 Bytes	477 363 800 Bytes	+32.8 KiB	+0.007%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2024-10-02 15:48:06 UTC Pre-commit check linux-x86_64-release-asan for 0830c29 has started.
⚪ 2024-10-02 15:48:10 UTC Artifacts will be uploaded here
⚪ 2024-10-02 15:51:14 UTC ya make is running...
🟡 2024-10-02 18:08:41 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
11011	10828	0	45	35	103

🟢 2024-10-02 18:09:37 UTC Build successful.
🟡 2024-10-02 18:10:09 UTC ydbd size 5.7 GiB changed* by +372.8 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 8030e8b	merge: 0830c29	diff	diff %
ydbd size	6 085 435 784 Bytes	6 085 817 480 Bytes	+372.8 KiB	+0.006%
ydbd stripped size	1 520 414 640 Bytes	1 520 510 032 Bytes	+93.2 KiB	+0.006%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}