Fix heartbeat channel subscriptions

[rmm5t]

• Automatically adds the heartbeat channel if not already subscribed
• Removes redundant keepalive ping on the socket

Fixes #113

[fb55]

Heartbeats are pretty noisy, so there should be a way to disable them to save bandwidth.

[rmm5t]

Heartbeats are pretty noisy...

🤔 Not compared the full or ticker or matches or level2 channel. The only channel that is less noisy than heartbeat is the user channel.

heartbeat is mild compared to anything else, and it comes with other very useful benefits (keeps the connection open, helps address lost sequences, etc).

Also, the longer-term intent of this automatic heartbeat addition is to eventually allow the WebsocketClient to automatically monitor the heartbeat, reconnect as necessary, and potentially fire an event when a missed sequence is recognized. i.e. The heartbeat does not need to necessarily propagate to the caller.

(I'll also rebase after the new conflicts.)

[fb55]

I like the change, just one comment

[rmm5t]

Rebased. Let me know if the automatic heartbeat channel is a show stopper. I'm in favor of keeping the automatic heartbeat. Without it, I suspect we'll need to add the keepalive socket ping back in (i.e all the deleted code from this pull-request and all the extra liability of 100+ lines of code that comes with it).

[rmm5t]

Rebased again to resolve new conflicts.

[fb55]

Thanks a lot @rmm5t!