This introduces a new HangTracker class that observers the run loop and notifies anything registered with it when the runloop is late. The idea is to eventually replace all uses of CADisplayLink with this. They are not exactly the same, display link works on vsync events and a runloop observer hooks directly into the runloop lifecycle.

In this PR I use the new HangTracker in the watchdog integration. This seemed like a pretty small use of display links and was a good opportunity to have a first crack at removing them. I also don't think this change requires any kind of new opt-in options, since it's all internal to how the integration works.

The SDK default behavior before this change is to enable the watchdog integration, which means by default the SDK spins the CPU in the background constantly waking up to process these vsync events. Since we are in the final stretch of quality quarter I wanted to get rid of this behavior and replace it with a passive observer, like this.

A few interesting implementation details:

• The hang tracker turns itself on and off by removing the runloop observer when nothing is subscribing to changes anymore.
• The majority of the code is Swift only, with a small ObjcBridge class wrapping the main implementation
• I used protocol based decency injection rather than subclassing, since this code can now be fully "swifty"

I also manually tested in the swift sample app by making sure when the debugger wasn't attached watchdog events still get sent when I force-quite the app with Xcode. Also verified that if I simulate a hang with the sample app and then force quit while the hang is in progress, watchdog events are not sent.