Plugins that are referenced as dependencies, but have not been
registered now will be imported. Newer plugins should register
themselves automatically, but older style plugins will be
explicitly registered after import.
The new system is backward compatible and will load older style plugins.
The new plugin framework allows plugins to track their dependencies, and
will auto-enable plugins as needed.
Dependencies are tracked via a class-level set named `dependencies` in
each plugin.
Plugin names are no longer tightly coupled with the plugin class name,
Pso EP8 style class names may be used.
Disabling plugins is now allowed, but ensuring proper cleanup is left to
the plugin implementation.
The use of a `post_init()` method is no longer needed for new style
plugins, but plugins following the old style will still require a
`post_init()` method.
For now, session_end is the same as disconnected, but once support is
added later for stream management, the two events will become distinct.
Plugins should add handlers for session_end for cleaning any session
state.
