Avoid repetitive directory scan during warmup.

We should avoid repetitive directory scan for each vbucket
during warmup. Instead, we can simply remove stale vbucket
database files by scanning a directory only once.

Change-Id: Ie153d6712407672b35b50359be929f434782d397
Reviewed-on: http://review.couchbase.org/37419
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>