Update chttpd_auth_cache for new couch_auth_cache

This updates the interface to chttpd_auth_cache to match the new
callback API expected by couch_httpd_auth. Specifically this allows for
the upgraded user documents to be written back to the correct user
database.

COUCHDB-2491
Merge branch '2491-refactor-couch-httpd-auth'

Fixes #13
Return error when open_revs=all and document doesn't exist

COUCHDB-2517
Re-indent to reflect recent addtion of a case branch

COUCHDB-2517
Merge branch '2517-open_revs=all-function_clause'

Fixes #15
Return ok:true on _bulk_doc update

Return `ok: true` on a successful _bulk_doc update like in
CouchDB 1.6.

Example success message on 2.0:
```
[{"id":"Brocket","rev":"6-b6fa9e703c3eb92aa9c3a49cedf1e8c2"}]
```

Example error message on 2.0:
```
[{"id":"Blaggie-System","error":"conflict",
  "reason":"Document update conflict."}]
```

Finally closes COUCHDB-2462
tests