Retry on backend error

[PierreF]

Port of jpmens/mosquitto-auth-plug#144

Allow to retry call to backend in case of error. This is useful if there are multiple backends behind a load-balancer and some backend are unhealthy (e.g. during canary deployment, where some backend use a new - bugged - version).

The default is to not retry, as this feature is not needed for every backend.

It's based on #120 because otherwise a conflict will occur. Since #120 contains lots of change, this PR should probably be only reviewed after #120 is merged (or only have a look at the 2nd commit).

This should be the last PR I made for this week :) Thanks for your works on this project, I hope all those PR won't give you too much work.

[PierreF]

This PR is based (or include) PR #120. I think once PR #120 got merged, the conflict will disappear. So doing nothing for now to avoid complicating git history.

[iegomez]

As mentioned, there are some discrepancies that should be solved with a rebase and force pushing the branch. I'll be able to properly review after that.