hydra-queue-runner: Improved scheduling

Each jobset now has a "scheduling share" that determines how much of
the build farm's time it is entitled to.  For instance, if a jobset
has 100 shares and the total number of shares of all jobsets is 1000,
it's entitled to 10% of the build farm's time.  When there is a free
build slot for a given system type, the queue runner will select the
jobset that is furthest below its scheduling share over a certain time
window (currently, the last day).  Withing that jobset, it will pick
the build with the highest priority.

So meta.schedulingPriority now only determines the order of builds
within a jobset, not between jobsets.  This makes it much easier to
prioritise one jobset over another (e.g. nixpkgs:trunk over
nixpkgs:stdenv).