Heartbeats algorithm:

1. Tunnelbroker after receiving any message from clients starts to wait for SOCKET_HEARTBEAT_TIMOUT time interval.
2. If there is SOCKET_HEARTBEAT_TIMOUT time without a message - send Heartbeat and start waiting again.
3. If Heartbeat was sent and there is another SOCKET_HEARTBEAT_TIMOUT time interval without any response - consider the connection unhealthy and close the socket.

From the client side:

1. If there is any message from Tunnelbroker start waiting CLIENT_SOCKET_HEARTBEAT_TIMOUT.
2. If there was more than CLIENT_SOCKET_HEARTBEAT_TIMOUT from the last message consider the connection unhealthy and close the socket.

CLIENT_SOCKET_HEARTBEAT_TIMOUT should be at least 2x SOCKET_HEARTBEAT_TIMOUT

This diff:

1. Adds new types
2. Adds logic for heartbeats
3. Implements tests exclusive for Heartbeats
4. Make sure heartbeats will not interfere with different tests

Depends on D9618

Tests