[IMP] skip builds when they will never be built

this marks builds as 'skipped' if a newer build in the same branch
arrives, or if they are too old to be built anyway
[FIX] make the 'rebuild' functionality work

if it is a skipped build, it will be put back in pending state, with
a high sequence number.

if it is a complete build, it will be duplicated
[FIX] correctly skip builds when they get too old
[IMP] improve runbot scheduler algorithm

Now it is very simple: it always builds sticky branch if any, otherwise
last pending job.
Merge pull request #12 from odoo/master-improvements-ged

[IMP] improvements to runbot scheduler