MB-24246: update highestDedupedSeqno when an existing value is changed

MB-24246: update highestDedupedSeqno when an existing value is changed

Without this, rangeReads are allowed to stop "too early" - potentially
missing out on items that have been updated, i.e.,

^ = HDDS (HighestDedupedSeqno)

 A₁   B₂   C₃            Initial items
[A₁   B₂   C₃]           rangeRead 1-3
[A₁   B₂   C₃]  B'₄       Update B
 A₁   B₂   C₃   B'₄       RR ends
[A₁   B₂   C₃]  B'₄       new rangeRead, still consistent
 A₁   B₂   C₃   B'₄       RR ends
 A₁        C₃   B'₄       purger removes stale B
!A₁        C₃!  B'₄       RR 1-3 could be requested, but would be
inconsistent. The HDDS serves to extend the end of a rangeRead to the
most recently deduped item, in this case B' replaced an older B.

Currently, this updates the HDDS too soon in the case of a stale item,
forcing the rangeRead to include the new version immediately even though
the stale item is still present and would still allow a valid range.
This is to be improved in a coming patch, in which the TombstonePurger
will update the HDDS when removing the stale item.

Change-Id: If6c57a86bab56ccc007b0fe17c9229218bb0c2c7
Reviewed-on: http://review.couchbase.org/77779
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>