Re-introduce cassim to the 2.0 codebase

[janl]

This was disabled for 2.0 because of subtle bugs.

The cassim Readme includes a bunch of notes for future direction here: https://github.com/apache/couchdb-cassim

Specifically, we need to pull CouchDB _security and _role handling from couch_httpd_auth into chttpd.

Benefits:

• performance improvements (30%+)
• clearer behaviour and avoidance of bugs in degraded cluster scenarios, e.g. Security objects cannot be synced if nodes are in maintenance mode #602