Change the C API around custom scheduler

[jedelbo]

What, How & Why?

Fixes #7094

☑️ ToDos

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

[nicola-cab]

thanks for fixing this.

[coveralls-official]

Pull Request Test Coverage Report for Build github_pull_request_282140

• 77 of 77 (100.0%) changed or added relevant lines in 2 files are covered.
• 210 unchanged lines in 19 files lost coverage.
• Overall coverage decreased (-0.07%) to 91.577%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/aggregate_ops.hpp	1	97.84%
src/realm/index_string.cpp	1	87.7%
src/realm/util/file.cpp	1	81.55%
src/realm/cluster.cpp	2	80.25%
src/realm/sync/network/websocket.cpp	2	75.15%
src/realm/sync/noinst/server/server_history.cpp	2	67.69%
src/realm/util/fifo_helper.cpp	2	85.71%
src/realm/uuid.cpp	2	97.06%
test/fuzz_group.cpp	2	55.65%
test/test_util_network.cpp	3	97.2%

Totals
Change from base Build 1796:	-0.07%
Covered Lines:	230815
Relevant Lines:	252046

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💛 - Coveralls