[IMP] improvements to runbot scheduler

Large change to the way the runbot schedule its next jobs.

Now, it has a moving window of (max-running) builds, and always build the last pending job (except that it build in priority sticky branchs).

If the amount of pending jobs is larger than this window, it will skip them.  But, if necessary, it is possible to unskip by clicking on the 'Force build' button in the dropdown menu.