Add websocket-level ping monitoring

[dblock]

Ping monitoring in slack-ruby-bot-server checks status of the bot, but faye/websocket-driver-ruby#61 (comment) is saying that we could be sending a ping at websocket level. Which should cause the next read to return a nil and abort an offline situation.

[ioquatix]

@dblock How are you going with this? I am back in NZ and can take a look at it if you still require assistance.

[dblock]

I am not doing any work rn with this. The replacement of celluloid with async has shipped, but did nothing to improve the underlying problem.

[RodneyU215]

I started to take a look at this last Friday and I believe I'll have a PR ready for review in a few hours. I'd welcome any feedback and guidance on it!

[RodneyU215]

@ioquatix think you'd be interested in reviewing and weighing in on my approach here #226? It's been a while since I've written in Ruby. 😅

[dblock]

Implemented for async-websocket in #226.

[dblock]

Implemented for Celluloid and EM in #236.