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limbo: handle volatile LSN lagging behind

The volatile confirmed LSN can legally lag behind the persisted
confirmed LSN, when the incoming CONFIRMations were done with a
smaller quorum somehow. Then the volatile LSN was still counting
acks, while the persisted LSN was moving forward on CONFIRM
entries.

Lets keep them in sync. The volatile LSN must be the present or
the future of the persisted LSN.

Otherwise the limbo worker saw the invariant broken and was
asserting.

The bug was only present in the Debug build, and it was
introduced in commit 516a3cc50
("limbo: collect ACKs on replicas").

Part of #8095

NO_DOC=bugfix
NO_CHANGELOG=the bug wasn't released yet

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