Cron jobs scheduling

Cron jobs are defined in the ir_cron table/model. This module deals with all
cron jobs, for all databases of a single OpenERP server instance.

It defines a single master thread that will spawn (a bounded number of)
threads to process individual cron jobs.

The thread runs forever, checking every 60 seconds for new
'database wake-ups'. It maintains a heapq of database wake-ups. At each
wake-up, it will call ir_cron._run_jobs_multithread() for the given database. _run_jobs_multithread
will check the jobs defined in the ir_cron table and spawn accordingly threads
to process them.

This module's behavior depends on the following configuration variable:
openerp.conf.max_cron_threads.