add an option to use randomized queues (or make that the default)

[h01ger]

currently rebuilderd queues packages alphabetically, which causes a side effect I'd like to be able to migate a bit:

if one runs two (or more) workers on one machine, often two or several similar packages are build at the same time. this is fine for python* packages but less good for libreoffice-* packages. if the queue order were randomized this would happen way less...

(and granted one should probably only have one worker per system, but then often still several systems run on the same physical host, so this still applies there, even though its better migated as well...)