Make couch-kvstore seqno midpoint rollback optimization configurable

Related to MB-47604 in which we need rollback to behave the same for both couchstore and magma for the sake of making comparisons between the two.

This optimization was added in the initial commit adding rollback support under MB-9900. No justification was given for it. In general I wouldn't expect it to have any impact as any customer running for more than a day or two will have run compaction and the rollback headers available will be for at least half of the total seqnos seen. However, as there was no justification for the optimization, simply removing it feels a bit risky, it may negatively impact some use case. As such, make it configurable satisfying the requirement for MB-47604.