Resolve "Duniter is using 100% of one CPU core"

Closes #297

The issue was that command_rx was always ready, even if it did not have received any command. As a consequence, the running async loop was heavily triggered causing high CPU usage.

Furthermore it always took precedence over handle_notification_event's future, which explains why peering documents were not shared.

@bgallois already noticed about a potential issue with command_rx which is only used for tests for now.

This MR also adds more logs.