Tor reachability monitor

This branch adds a component that monitors the state of the Tor plugin and calls observers when the plugin has been continuously active for 10 minutes. After 10 minutes we assume that our hidden service is reachable and contacts should be able to connect to us directly.

The reachability monitor will be started and destroyed by the mailbox client manager (#2228 (closed)).

Infrastructure for #2292 (closed) and other mailbox workers.