Merge bitcoin/bitcoin#30517: refactor: Add FlatFileSeq member variabl… #2648

Job	Run time
Win64 native, VS 2022	1h 54m 51s
ASan + LSan + UBSan + integer, no depends, USDT	1h 22m 50s
macOS 13 native, x86_64, no depends, sqlite only, gui	1h 1m 52s
test each commit	0s
	4h 19m 33s

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