Add at_exit hook for Tracer shutdown

[marcotc]

#820 was merged into the old release branch.
This PR applies that same changeset to master instead.

Original description:

When traced Ruby processes are terminated, they may not permit the worker an opportunity to flush existing traces. This pull request adds an at_exit hook that calls shutdown! on the Tracer, so it can be cleaned up in a more graceful fashion.

[delner]

👍