MB-12483: Make sure vbucket_state initializes all variables

We need to make sure that this data is always completely filled
out so that we don't accidentally write garbage data to disk.

Change-Id: I196e7ca9f5bada8e0df90ddb01b6e952650bed56
Reviewed-on: http://review.couchbase.org/42729
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>