Add synchronous jobs

[mbanck-cd]

We needed a possibility to execute some jobs / queries synchronously every time /metrics is requested. So I added the infrastructure to execute jobs with an interval <= 0 synchronously when the http-handler is called.

interval: '0s' # an interval <= 0 will make the queries synchronous

This is a rebase and re-submission from #10 which erroneously got marked as merged a while ago.

[mbanck-cd]

Did somebody take a look at this?

[dewey]

Hey @mbanck-ntap, I don't really have time to test it, but it looks good to me and as everything is limited to the 0 case it shouldn't cause any issues for people not using it. Could you rebase on master then I could merge it and create a release.

Thanks!