CMDP: Add Notification System

This MR extends the CMDP protocol by a third message type, notifications.

Notifications are meant for distributing metadata on the individual topics of each message type:

• List of log topics available, along with a description
• List of registered metrics along with their description

Notification messages are sent to every remote subscribed to LOG? and STAT?, respectively. They are sent

• upon subscription of a new remote
• whenever the list of log topics or registered metrics changes

Open things:

• CHIRP manager fluffs up the tests
• Need to converge on how to handle topics on UI side, see discussion below
• Need manager locator