MB-100 prevent couch_stats_reader from spamming logs

on bucket creation.
couch_stats_reader was printing 2 log messages for each
VBucket file that was not yet created on disk

Change-Id: I065259289b2c8b19a199130cffaad077a5d78bcc
Reviewed-on: http://review.couchbase.org/29000
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>