Reuse transaction when refreshing subscriptions after commit

[jbreams]

What, How & Why?

Instead of throwing the read transaction away after committing subscription changes and then opening a new frozen transaction at the committed transaction's version - we should just re-use the read transaction.

☑️ 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 jonathan.reams_3582

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 61 unchanged lines in 13 files lost coverage.
• Overall coverage remained the same at 91.132%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/array_string.cpp	1	88.03%
src/realm/util/compression.cpp	1	89.62%
test/fuzz_tester.hpp	1	57.73%
src/realm/cluster.cpp	2	75.85%
src/realm/sync/noinst/client_impl_base.cpp	2	82.52%
src/realm/table_view.cpp	2	92.99%
src/realm/sync/noinst/protocol_codec.hpp	3	76.27%
src/realm/sync/noinst/server/server.cpp	3	74.61%
src/realm/link_translator.cpp	4	76.92%
src/realm/sync/transform.cpp	4	60.95%

Totals
Change from base Build 2648:	0.0%
Covered Lines:	217398
Relevant Lines:	238554

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💛 - Coveralls