[Ready for Review / Merge] Subtree/Submodule stripe/dagon

[daniel-sudz]

• [x] copy dagon src into scalding module
• [x] dagon tests pass in isolation
• [x] wire scalding-dagon to CI tests
• [x] re-write scalding imports against module
• [x] clean up PR

notes: dagon from the repo used scalatestplus that had conflicts with the scalatest and scalacheck versions of scalding default settings. I removed scalatestplus and replaced forAll mixin with import org.scalatest.prop.GeneratorDrivenPropertyChecks._. I have no idea if this is correct I am not really familiar with the scalatest/scalacheck ecosystem.

I also had to tweak the src a bit due to lint errors (touching up sealed property). I'm doing to re-order the commit diffs for this to be transparent.

https://github.com/twitter/scalding/issues/1978

[daniel-sudz]

@johnynek CI looks good on the last couple of commits. I just did some touch up cleanup I think it's ready for merge in general

edit: except that one flaky test that we have an issue for but unrelated