standard uses postgresql sequences, no locks, gaps are allowed.
no_gap uses an SELECT FOR UPDATE NO WAIT, no gaps are allowed but an exception could be raised if the lock is unavailable.
we also deprecated the infamous get()/get_id() api in favor of next_by_code()/next_by_id()
the specs of this change were inspired by openlabs sequence-postgres module
bzr revid: al@openerp.com-
20110930190053-ovd4qlwt7hll02ch