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