[FIX] crm, hr_recruitment, task, issue: fixed default priorities + order based on priority (desc for everyone now)
[IMP] models: move prefetching of records back to method _prefetch_field

The selection of records in cache for prefetching was moved to method
_read_from_database() by xmo at rev 785018cc in order to fix an access right
bug.  But this introduced an issue: to explicitly avoid prefetching, you should
use read() instead of browsing records.  We revert the change by xmo, without
reintroducing the bug (which apparently was fixed by another way).
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64