Interval based pulling

[nmandery]

Allow layers to be pulled in an configurable interval.

In the most minimal case this would me mean adding a pull.job to the queue every n-seconds. In the best case batyr should estimate the total duration which it will take to finish all jobs which are queued and add the new pull event to the queue so that the interval is kept with as little delay as possible.

The second approach would mean collecting statistics on the duration of all job types and variants