Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

async_wrap, async_hooks and a little bit of everything else

This PR supersedes my other PR for async_hooks support. The implementation has been cleaned up and @thlorenz has helped create much better tests (though they may need to be changed from their current location) and which accounts for ~3300 lines of the change. Documentation does exist, but needs to be cleaned up some before it's included. The code is ready for initial review.

For anyone reviewing, I carefully went through and divided all the changes into logical commits that should all pass. So while reviewing feel free to use each commit individually. Though GitHub fails at displaying them in the correct order.

The one aspect of this PR that I don't like but have gone through hell trying to get around to no avail is the usage of initTriggerId(). It's basically a way to propagate the "triggerId" to a constructor. The code comments explain some of those difficulties, but I hope someone can come up with something better.

@nodejs/ctc

Refs: #8531

CI: https://ci.nodejs.org/job/node-test-commit/8503/