This is a comments-only change to note the threadsafety properties. It may be the case that ForAttempt is always threadsafe, but the Go race detector complains about the default setting on lines 44-52. And the workaround is quite easy.
Amazon recently wrote about performance gains of using jitter in
concurrent systems. http://www.awsarchitectureblog.com/2015/03/backoff.html
Seemed simple enough to add and wont change any code that may already be
using Backoff.