MB-10502: adjust vbucket-seqno stats and unbreak xdcr

New-style checkpointing is using this stat and recent ep-engine change
requires different stat name.

Change-Id: Idc5674359ab2999f8afa907910765d9d6267e379
Reviewed-on: http://review.couchbase.org/34668
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>