Change the SQL service to a lightweight SQL API

The "SQL service" allows plugins to execute SQL queries on the host server.
But it actually exports a subset of the libmysql client API meant for bundled clients to connect to arbitrary servers.
This service could have exported more direct interfaces to the SQL layer instead of hacking them into a remote connector like a plugin.

It's highly unlikely that a plugin needs to connect to servers other than the one it's plugged into, but those cases should use libmysql (or better, Connector/C or Connector/C++) directly.