MB-12483: Remove the default constructor in the vbucket_state struct

The default constructor can leave uninitialized fields and we persist
this structure to disk. In oreder to prevent garbage from being written
we should remove the default constructor.

Change-Id: I028fac1dc112bb454779a30f695eb180278455df
Reviewed-on: http://review.couchbase.org/42728
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>