We had been implicitly assuming that clients send us sorted groups, but
unsurprisingly that's not always the case. The additional sorting here
should be redundant, but the consequences of merging unsorted groups are
severe -- we can end up with uniqueness violations on the primary key in
the database -- and so we add an additional sort here.

COUCHDB-2735

Document buckets are sorted by docid, but the validation code was
reversing the buckets. If multiple clients send concurrent updates for
the same document the broken sorting can result in duplicate documents.

The particular structure of the patch here is chosen to match the 2.x
codebase.

COUCHDB-2735

The documentation was incorrect insofar that it only described its
functionality for client verification, although the configuration is
used for server verification as well.

- closes COUCHDB-2298
- closes COUCHDB-2299

Admin users are stored in .ini files and are not full-fledged user
documents. Internally, a fake document is made to allow insertion into
the auth cache. CouchDB 1.6 introduced a feature to upgrade password
hashes from the legacy simple hash scheme to the stronger PBKDF2
scheme. It inappropriately attempted to do this to the fake admin
docs, which do not pass the _design/_auth validation checks. This is
fortunate, however, as CouchDB would then have written the admin users
into the users database causing widespread confusion and fear.