MB-8713: set replica building vbuckets to state pending

As requested by ep-engine folks. So that they can preserve resident
ratio of future-master vbuckets.

Change-Id: I846ef2c7dd5ab39f9f7a6b52e1d1db1f5699fe02
Reviewed-on: http://review.couchbase.org/28086
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>