VPN improvements

[Darkren]

Did you run make format && make check?
Yes
Fixes #473

Changes:

• Packet of type NetworkProbe added. Packet handling goes this way. Once an interval of time (3 seconds now) route group sends network probe to the remote. Packet includes timestamp of when the packet was sent and a throughput of the remote. Throughput is an amount of bytes per second successfully delivered to the remote. So the source route group tells remote the throughput of the remote. Upon receiving this packet remote calculates latency based on the timestamp in a packet;
• Amount of bandwidth sent/receiced is calculated on each sent/received packet;
• NoiseRouteGroup and SkywireConn are exposed to allow outer code get RG's metrics;
• IsAlive method is added to route group. RG is considered alive as soon as underlying route is functional;
• Method to fetch connections metrics for a specific app is added to hypervisor.

How to test this PR:

1. Run any app. VPN for example
2. Request data from hypervisor, like this:

http://{HYPERVISOR_URL}/api/visors/{VISOR_PK}/apps/{APP_NAME}/connections

[nkryuchkov]

Good job! Works great.

[jdknives]

Very smooth. Please respond to the request by Nikita and change target branch to v0.4.0 and then this can go.