add freelist verification at more points during commit

[finnschiermer]

Improve checks for free list corruptions

• Enable some checks in release mode, which where otherwise only present in debug mode.
• Run check for overlapping free blocks (free list corruption) in more places during commit.

This is in the hope of getting back better clues about the root cause of reported free list corruptions.

[finnschiermer]

This is no longer needed.

[coveralls-official]

Pull Request Test Coverage Report for Build github_pull_request_279261

• 124 of 124 (100.0%) changed or added relevant lines in 2 files are covered.
• 29 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+0.06%) to 91.624%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/object-store/sync/sync_manager.cpp	2	86.96%
src/realm/sync/noinst/client_impl_base.cpp	2	85.78%
src/realm/sync/noinst/server/server.cpp	2	75.83%
src/realm/table_view.cpp	2	94.18%
src/realm/sync/noinst/protocol_codec.hpp	3	76.21%
src/realm/util/future.hpp	3	95.81%
test/fuzz_group.cpp	3	51.61%
test/test_thread.cpp	3	66.67%
src/realm/util/file.cpp	4	81.73%
src/realm/util/fifo_helper.cpp	5	85.71%

Totals
Change from base Build 1745:	0.06%
Covered Lines:	230660
Relevant Lines:	251746

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💛 - Coveralls