[IMP] WorkerCron: make the cron workers not all starting at the first database. Better repartition of work
[IMP] WorkerCron: renice process

Backport of commits 4891 chs@openerp.com-20130701120345-4687avu7re5pw6sf and 4892 chs@openerp.com-20130701125548-sgcaf71avwpleml1
of saas-1 branch