Refactor Stoppable interface

[carlhua]

Summary

Currently, the Stoppable interface requires inheritance and constructor parameters, which makes messy declarations of abstract base classes that implement Stoppable. We need to change the way the Stoppable hierarchy is formed to eliminate the need for constructor arguments.

Exported from RIPD-118

[scottschurr]

@thejohnfreeman, since 78245a0 was merged in 1.7.0-b10, can this issue be closed?

[thejohnfreeman]

That was a minor cleanup commit. The real refactor is #3741, which is presently awaiting resolution of merge conflicts. I don't mind closing this issue now, or waiting until that PR is merged.

[scottschurr]

Seems good to wait until #3741 is merged, just in case something unexpected happens.

[scottschurr]

@thejohnfreeman, I no longer see #3741 as an open pull request. It looks like it was merged in 1.8.0-b3. Are we good to close this issue?

[thejohnfreeman]

Indeed!