Make stale connection detection configurable

[mbrevoort]

A Beep Boop user ran into an issue when on connection and periodic points in time, they were doing A LOT of work that was maxing out CPU so much that it caused node.js's event loop to queue excessively. RTM connections were healthy and returning pong responses but the node process wasn't processing the pong response in time because the event queue was so backed up. This caused a lot of thrashing of connects/discconects/connects within their process:

Ultimately they need to better manage/throttle the work being done inside of the process, but the mitigation to avoid the connection thrashing was to increase the amount of time considered to consider a connection stale.

An overview of the changes:

• add stale_connection_timeout to make stale connection detection configurable. This is the number of milliseconds to wait for a pong response to a ping
• replace the use of an interval with a timeout to ensure we don't clog the event loop under duress.
• add details to the Slack Readme

[jonchurch]

Hey this looks good! It's a weird edge case for sure, but seems reasonable.

Any reason you took out the PING SENT and PONG RECEIVED debug messages?