improvement of relay related metrics; remove relay clients if we have no more connections

[RolandSherwin]

• Remove relay clients from the SwarmDriver::connected_relay_clients tracker if the Reservation has been closed. This happens when all the connection with the relay client has been lost. Libp2p does not have an event for this yet, so we have to do a manual check for this.
• Calculate the peers_in_rt value directly from the kbuckets instead of acting upon the libp2p events. The latter gave incorrect values, and it is very prone to errors, as everyone should be careful to call the update fn everytime we manually remove a peer from the RT.
• Add connected_relay_clients metric.
• Add relay_peers_in_routing_table, peers_in_non_full_buckets, relay_peers_in_non_full_buckets and percentage_of_relay_peers metrics.