MB-12365: Initialize all variables in CouchKVStoreStats

Change-Id: Ifdd1409a6bd4e445f2ef7cce98a73ac2e8cbbd47
Reviewed-on: http://review.couchbase.org/42334
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>