Improve TsVbuuid performance

TsVbuuid can be indexed directly like an array rather than to
iterate fully every time. The Vbnos field has been removed.
The position implies the vbno now.

Change-Id: Ic537ab7e8514dab0a4c1fcec912e2c89a6805983
Reviewed-on: http://review.couchbase.org/41605
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>