Introduce subscription IDs (#828)

This PR implements subscription IDs which can be used to unsubscribe from events. The specification is refactored
to operate on a list of subscriptions.

Co-authored-by: jgraham <james@hoppipolla.co.uk>
Co-authored-by: Maksim Sadym <69349599+sadym-chromium@users.noreply.github.com>