Disallow backoff.Config.MinBackoff to be zero

[tpaschalis]

Currently, we don't perform any kind of validation in the configuration that is passed to create a new Backoff object.

Passing a minBackoff of zero means that both nextDelayMin and nextDelayMax will be set to zero, meaning that NextDelay is also always zero and that we'll never wait for a meaningful amount of time.

At the very least we can document this kind of behavior, but I believe that adding some rudimentary validation of the configuration passed shouldn't be too much overhead.