Remove exec from Watches, add Status(Un)Healthy things Watch publishes

[tgross]

This PR removes the onChange field from watches per the design in RFD86. (Well, ok, it removes the field from backends because I haven't made that last config change yet). A user who wants the effect of the CPv2 onChange handler will create a job with a configuration like:

jobs: [{
  name: "onChange",
  when: {
    source: "watch.upstreamA",
    event: "changed", // this can be "healthy" or "unhealthy" now too!
  },
  exec: "/bin/onChange.sh"
}],
watches: [{
  name: "upstreamA",
  poll: 5
}]

There are several examples of this in both the unit tests and the integration tests.

Note that this PR currently includes commits from #312 and as usual I'll rebase this once that's been merged for easier review. 3df4e2f is the only commit that will be in this PR when that's done. I'm also feeling like I missed something even though all the tests pass (locally at least), so I want to take one more pass through anyways. Good-to-go.

cc @cheapRoc @geek @jasonpincin @misterbisson