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>
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>