juju/retry take 2 - looping

[Tim Penhey]

Hi folks,

https://github.com/juju/retry/pull/5/files

As often is the case, the pure solution is not always the best. What 
seemed initially like the best approach didn't end up that way.

Both Katherine and Roger had other retry proposals that got me thinking 
about changes to the juju/retry package. The stale-mate from the tech 
board made me want to try another approach that I thought about while 
walking the dog today.

I wanted the security and fall-back of validation of the various looping 
attributes, while making the call site much more obvious.
The pull request has the result of this attempt.

It is by no means perfect, but an improvement I think. I was able to 
trivially reimplement retry.Call with the retry.Loop concept with no 
test changes.

The tests are probably the best way to look at the usage.

Tim