read/write same thread local storage

[nicola-cab]

What, How & Why?

Write/Read same thread local storage and pass user code data back to the right thread.

☑️ ToDos

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

[coveralls-official]

Pull Request Test Coverage Report for Build nicola.cabiddu_1205

• 16 of 16 (100.0%) changed or added relevant lines in 4 files are covered.
• 80 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.009%) to 91.711%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/object-store/shared_realm.cpp	2	93.04%
src/realm/sync/noinst/server/server_history.cpp	2	67.94%
src/realm/table_view.cpp	2	94.18%
src/realm/sync/noinst/protocol_codec.hpp	3	76.72%
src/realm/util/serializer.cpp	3	90.03%
src/realm/sync/noinst/server/server.cpp	4	76.83%
src/realm/util/assert.hpp	4	87.1%
src/realm/index_string.cpp	6	87.81%
src/realm/cluster_tree.cpp	7	91.8%
src/realm/sync/instruction_applier.cpp	7	70.26%

Totals
Change from base Build 1907:	0.009%
Covered Lines:	232225
Relevant Lines:	253214

---

💛 - Coveralls

[nicola-cab]

I don't know if there is any interest in pursuing this PR, so I am closing this for now. In case we decide, we want to explore this path, we can always re-open it.