dropped some code paths that could open arbitrary vbucket

For places where we open master vbucket, I've created brand new
functions.

Change-Id: I282715cb8393b5b2e8b2becdadd6eb7f1ebb80f2
Reviewed-on: http://review.couchbase.org/39511
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
prevented direct vbucket access via capi in few places

We had _changes and even some bits of CRUD against vbucket still
exposed.

Change-Id: If7261aa011948f135ba3119348427e9b7af4a549
Reviewed-on: http://review.couchbase.org/39512
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
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>