Implement SQL Registry

[richardartoul]

A SQL registry that works with MySQL/PostGres, etc would make it much easier for people to start leveraging NOLA since most people don't know how to run/administer an FDB cluster. In addition, this would make it trivial to use NOLA in cloud environments with hosted solutions like RDS.

Ideally we would create a "real" implementation of the Registry interface backed by SQL so we could see the different tables and query them for debugging etc. However, the quick and dirty solution would be to implement this small interface and just treat the SQL database as a dumb KV store.