Add a configuration option to open a channel for metadata

[diogob]

This is an attempt to create a feature that can be used to implement both #41 and #52.

The feature behaviour can be summarized as:

1. A configuration option may define a channel used for sending metadata
2. Once this configuration is enabled that channel allow only read sessions to be open.
3. Once this configuration is enabled all meta-data is sent to that channel on internal state changes through a PostgreSQL notification. Initial implementation would send messages on new websocket sessions. The metadata for the initial implementation is the JWT claims and the Multiplexer state.

This would allow executing custom functions in the database provided you have a process listening on the postgres-websockets listen channel that can call custom functions (see pg-recorder for an example).

It can also be used to detect state changes in web clients that have a proper token to read the meta-data channel.

The number of connections in applications like a chat can be inferred just by the number of listeners in the Multiplexer.

To get the number of connections in more complex scenarios (with write-only sessions) we would need to track websockets connections closing (which won't be a part of this PR).