Reschedule the vbucket bg fetch task if its DB file is not created

We might receive getMeta requests for non-existent keys before
their vbucket DB file is created. Therefore, it is required to
reschedule a bg fetch job until the file is created.

Change-Id: I4c692526fceea3b0015728ec40d9d815bc58ff6b
Reviewed-on: http://review.couchbase.org/33221
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>