[MERGE] ir_sequence standard and no_gap implementation by Thu (vmt)

[MERGE] ir_sequence standard and no_gap implementation by Thu (vmt)
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