MB-6334: complete dropping support for seeing ddocs via _all_docs
MB-6334: complete dropping support for seeing ddocs via _all_docs

Since some time ago ddocs namespace is supposed to be totally
separate. Otherwise we'd need key mangling to 'carve _design/ and
_local hole in our visible-to-memcached keyspace' which we decided not
to have.

Thus sooner we change our implementation to reflect that decision, the
better.

Change-Id: Ied392ee3320b1aee12e6d78e5d541b59fd38e1af
Reviewed-on: http://review.couchbase.org/19914
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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