MB-9693 Check the vbucket existence only upon a bg fetch completion

As there is always a race in vbucket state changes, we should check
the vbucket existence only upon completing a bg fetch and then let
a memcached worker thread visit the vbucket again to complete GET
request.

Change-Id: I9cdc76897c5e18f4c02a75606ec75a3f4dcee063
Reviewed-on: http://review.couchbase.org/32471
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>