Add subscription to heartbeat channel on GDAX

[chuggol]

I was not able to make GDAX trade subscription sustainable - for me it terminates every other day without any indication - websocket does not disconnect.
You can emulate this by disconnecting the network after initial subscription has been established.

Checking if I've received any messages in the last n seconds would work, but seems hacky.

GDAX API docs indicate there is heartbeat channel to verify that connection is healthy, would be great to have that implemented.
https://docs.gdax.com, search for 'heartbeat'

[dozd]

There is a new isAlive method in the streaming service - does it work?

Heartbeat pattern seems nice but someone need to make a research whether it's commonly supported feature by exchanges or GDAX is the only one.

[nihiluis]

I get the same disconnects. Some callback on disconnect would be appropriate. I think there also was some PR for automatic reconnect?