Add lock on Retry

[Jleagle]

This fixes a race condition that can be caused by a package re-using a BackOff.

If Retry() is called twice with the same instance of a BackOff, one may Reset() the other.

This was found using Go's -race flag. Thanks.

[coveralls]

Coverage increased (+0.3%) to 85.106% when pulling ebeb868 on Jleagle:master into 1e4cf3d on cenkalti:master.

[cenkalti]

You can use your own mutex. I don't want to put a mutex into the package.

[Jleagle]

👍