bugfix: multiple key-versions for same seqno.

Shall be encapsulated within same VbKeyVersions.

Change-Id: I6b1f0853f5e58a7e3eb11a022b63e2405a0f0f39
Reviewed-on: http://review.couchbase.org/43484
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>