retry: use time.After rather than manage timer lifecycle
retry: use time.After rather than manage timer lifecycle

After some analysis of stdlib, we found that time.After is sufficient to
avoid leaking a goroutine. This is because the send on the time channel
is actaully non-blocking.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

docker/go-events