refactor relay threads

[jeffthibault]

Relay class

• Add thread instantiation to connect method
• Add is_connected method which checks if the ws.run_forever thread is alive. I think this is more reliable because this thread will be torn down on disconnect.
• Various renaming

Relay Manager

• Remove thread instantiation from add_relay (handled in relay.connect now)
• Add relay_connection_monitor thread, which periodically (connection_monitor_interval_secs) checks if the relays are disconnected and reconnects them if they are.

Note: I felt like reconnection logic belongs in the relay manager as it's job is to manage the relays