Store configuration in Server

MaxScale should have an option to maintain all of its configuration in the Server deployment it sits in front of.

It could store JSON or YAML files in TEXT columns, or it could normalize the data in a set of system tables. As a result, MaxScale itself becomes stateless and its configuration can be managed either by the UI or directly in Server by editing the data in the system tables.

This is particularly helpful in the context of SkySQL. If a MaxScale instance fails, a new one can be brought up and it can pull its configuration from the primary Server instance.

There is the challenge of seeding the Server definitions though. One option is for SkySQL to use the MaxScale REST API to seed the Servers. MaxScale can then "register itself" with the primary Server instance, storing its configuration and information there. For on premises, they would have to be added to the local config file. In fact, MaxScale could continue to use the local config file, but build and update it using data queried from Server.

Additionally, it might be nice if Server instances could send queries to a MaxScale admin port. This port isn't backed by a database, but rather by MaxScale itself. We might be able to add MaxScale as a server definition in Server and use it to send commands to MaxScale such as "refresh configuration" or "update xyz configuration". This would help synchronous the configuration of multiple MaxScale instances. Alternatively, MaxScale could refresh its configuration every X seconds or have a button in its UI to refresh of its configuration.

The primary objective is to make MaxScale stateless and to externalize its configuration.