This PR adds a SessionID field to the ipn.Notify struct so that
ipn buses can identify a session and register deferred clean up
code in the future. The first use case this is for is to be able to
tie foreground serve configs to a specific watch session and ensure
its clean up when a connection is closed.
Updates #8489
Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>