Support other databases (SQLite, NoSQL)

[msimerson]

Add support for other database engines. This requires:

• removing all MySQL specific ENUM columns (mostly done as of v2.10)
• Support NoSQL dbs like Redis and MongoDB

[msimerson]

This is a version 3 feature, and current intentions are to support the following DBs: none/directory of zone files, mysql/mariadb, and at least one document store (MongoDB or Elasticsearch).