Test a connection through the gateway (6 per minute)

The server sends one request through the gateway to its own echo endpoint (10 s timeout) and reports whether it connected, the exit address it saw and the latency. exit_country is a country observed for that exit address, never the requested one: when none was observed it is null and country_verified is false, so look the exit address up yourself before relying on the country. ok: false carries the gateway error_code (for example E_NO_STOCK_COUNTRY, E_CAP_EXCEEDED, timeout). The test uses a few kilobytes of the balance. In development with the mock upstream the result is simulated (simulated: true).