bind this reference

[brendanmcfarland]

Looking to address an error we've seen on the heartbeat:

(node:1) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'queueError' of undefined
    at Timeout.heartbeat [as _onTimeout] (/snapshot/ecs-watchbot/lib/message.js:82:19)
    at ontimeout (timers.js:498:11)
    at tryOnTimeout (timers.js:323:5)
    at Timer.listOnTimeout (timers.js:290:5)

Looks related to how setInterval handles this

cc: @jakepruitt

[brendanmcfarland]

@jakepruitt mind giving this a quick review - was able to replicate the error above with master, and this fix seems to have eliminated the undefined this. Worked with the initial message a worker processed and each of the subsequent ones.

[jakepruitt]

Sweet!