Avoid relying on refcount for mediator bucket cleanup

[snaury]

Changelog entry

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Changelog category

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Description for reviewers

I noticed that timecast service used bucket entry refcount to make some decisions, which are leftovers of the old behavior before the rewrite (e.g. the case for RefCount() == 0 cannot even happen anymore). Check for Tablets.empty() explicitly instead, and clear waiters consistently on either update or reconnect.

Related to #14029.

[github-actions]

🟢 2025-02-19 15:43:02 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-02-19 15:44:09 UTC Pre-commit check linux-x86_64-relwithdebinfo for 80bbc77 has started.
⚪ 2025-02-19 15:44:20 UTC Artifacts will be uploaded here
⚪ 2025-02-19 15:47:08 UTC ya make is running...
🔴 2025-02-19 15:49:26 UTC Build failed, see the logs. Also see fail summary

[github-actions]

⚪ 2025-02-19 15:44:11 UTC Pre-commit check linux-x86_64-release-asan for 80bbc77 has started.
⚪ 2025-02-19 15:44:16 UTC Artifacts will be uploaded here
⚪ 2025-02-19 15:47:15 UTC ya make is running...
🔴 2025-02-19 15:49:39 UTC Build failed, see the logs. Also see fail summary

[github-actions]

⚪ 2025-02-19 16:08:28 UTC Pre-commit check linux-x86_64-relwithdebinfo for 17134f9 has started.
⚪ 2025-02-19 16:08:44 UTC Artifacts will be uploaded here
⚪ 2025-02-19 16:11:39 UTC ya make is running...
🟢 2025-02-19 17:10:05 UTC Tests successful.

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
19048	17703	0	0	1214	131

🟢 2025-02-19 17:12:33 UTC Build successful.
🟢 2025-02-19 17:12:51 UTC ydbd size 2.1 GiB changed* by -3.0 KiB, which is <= 0 Bytes vs main: OK

ydbd size dash	main: 02fadfa	merge: 17134f9	diff	diff %
ydbd size	2 238 945 296 Bytes	2 238 942 248 Bytes	-3.0 KiB	-0.000%
ydbd stripped size	474 411 320 Bytes	474 410 552 Bytes	-768 Bytes	-0.000%

^{*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]

⚪ 2025-02-20 07:49:55 UTC Pre-commit check linux-x86_64-release-asan for 17134f9 has started.
⚪ 2025-02-20 07:54:17 UTC Artifacts will be uploaded here
⚪ 2025-02-20 07:57:15 UTC ya make is running...
🟡 2025-02-20 08:58:10 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Details

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
11736	11630	0	36	34	36

⚪ 2025-02-20 08:59:16 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-02-20 09:17:31 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Details

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
159 (only retried tests)	121	0	1	5	32

⚪ 2025-02-20 09:17:41 UTC ya make is running... (failed tests rerun, try 3)
🟢 2025-02-20 09:29:26 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
65 (only retried tests)	30	0	0	4	31

🟢 2025-02-20 09:29:33 UTC Build successful.
🟢 2025-02-20 09:30:05 UTC ydbd size 3.6 GiB changed* by -9.5 KiB, which is <= 0 Bytes vs main: OK

ydbd size dash	main: 02fadfa	merge: 17134f9	diff	diff %
ydbd size	3 894 661 032 Bytes	3 894 651 280 Bytes	-9.5 KiB	-0.000%
ydbd stripped size	1 363 540 128 Bytes	1 363 533 792 Bytes	-6.2 KiB	-0.000%

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