Performance Audit 2017: Beacon: Breakup page_ready callback

[nicjansma]

Each plugin may register for callbacks such as page_ready to add its data to the beacon.

When Boomerang fires the page_ready event to all listeners, it executes each serially.

We may want to investigate breaking up the callbacks (via setImmediate or something) to ensure long-running callbacks aren't all executed synchronously.