don't open actual vbuckets for /<bucket>%2F<vbucket/<rest> reqs

CAPI mode of XDCR is still using theses for _bulk_docs and
_get_missing_revs. And remains of old "lets provide couch api at least
for vbuckets" code was still opening those vbuckets. Simply because
couchdb frontend api (which we introduced extractly to hook into couch
http layer) is expecting open database.

So just like with bucket access we now fake couch db instance.

Change-Id: I4186786dc2793b1c6c91fca264143bdc8e4bb3bf
Reviewed-on: http://review.couchbase.org/39513
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>