MB-11228: start LastSeenSeqno with StartSeqno

Most of the time it doesn't matter because we actually process at
least one mutation (which update LastSeenSeqno) before processing end
stream. But in case we see no mutations (in case StartSeqno =:=
EndSeqno), then we'd otherwise signal LastSeenSeqno =:= undefined and
break parent xdc_vbucket_rep which would set current snapshot seqnos
to undefined.

Change-Id: I05bc70742b7880e5751e44fc57cb89d1db3fdc8e
Reviewed-on: http://review.couchbase.org/37697
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>