Feature/gateway monitoring

[jstuczyn]

This change allows to monitor the gateways to see if they're up. I'm not entirely sure how well it's going to scale into hundreds of gateways, but it seems to work rather well for the current testnet.

Closes #384

The current limitations that come to my mind are:

• we do not test the available storage on gateways - we maintain persistent connections to the gateways so in theory the gateways might appear to be working yet be incapable of storing offline traffic
• we never generate any new keys so in the long run the gateways would be able to figure out that we send the test packets

Do note that to deploy it, we probably need to adjust the validator code first.