Currently Connector API is public API (what seems unnecessary as one can
query connectors by ID), also there are some static singleton services
that binds consumers to that explicit class and might hinder future
evolution.

This now makes all connector API provisional except for one shared with
another connector but reducing that part as much as possible. Also some
interfaces are extracted and registered as services to allow loose
coupling. In one case the static access can be even avoided completely
now.