Cancel subscription notifications

[danieltabacaru]

What, How & Why?

Completion callbacks are canceled when a fatal error is received from the server or the sync session becomes inactive. The subscription notifications would not fulfill in such cases causing a hang for the users. This PR addresses that by canceling the subscription notifications in such cases.

Fixes #5315.

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• [ ] C-API, if public C++ API changed.

[coveralls-official]

Pull Request Test Coverage Report for Build daniel.tabacaru_599

• 85 of 87 (97.7%) changed or added relevant lines in 3 files are covered.
• 87 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.01%) to 91.572%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
test/object-store/sync/flx_sync.cpp	72	74	97.3%

Files with Coverage Reduction	New Missed Lines	%
src/realm/index_string.hpp	1	82.86%
src/realm/array_string.cpp	2	87.78%
src/realm/object-store/sync/sync_manager.cpp	2	86.58%
src/realm/query_expression.hpp	2	93.51%
src/realm/sync/instruction_applier.cpp	2	70.21%
src/realm/sync/noinst/server/server_history.cpp	2	67.69%
src/realm/util/file.cpp	2	81.25%
src/realm/sync/noinst/client_reset_operation.cpp	4	87.23%
src/realm/sync/noinst/server/server.cpp	4	76.25%
src/realm/util/assert.hpp	4	87.5%

Totals
Change from base Build 1792:	-0.01%
Covered Lines:	230759
Relevant Lines:	251998

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💛 - Coveralls

[michael-wb]

Looks good - maybe you could rename cancel_subscription_notifications to cancel_sub_notifications