Integrate ActiveJob / DeliverLater / GlobalID with Rails

WIP.
As far as I can see from the source, `LogSubscriber` does not call any method on itself when instrumentation starts, just when it finishes: [source](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/subscriber.rb#L85) (`start` and `finish` are called by `Notifications` before and after the block is yielded). We want a separate log message when the job starts, which at this point requires a separate instrumentation.

I wouldn't mind changing `LogSubscriber.start` to call `#{name}_start` if that method exists, but that's not backward compatible if people are already using their own `_start` event.

/cc @tenderlove