[IMP] orm._auto_init: avoid copying VARCHAR column when increasing size, alter in...

[IMP] orm._auto_init: avoid copying VARCHAR column when increasing size, alter in-place instead + better temporary names

Historically, we used a copy+rename technique during
table upgrades to alter varchar columns sizes.
This was only necessary to permit downsizing
columns, which we do not want to do anymore (it is
too dangerous, so has to be done explicitly when
required).

Switching to a pure ALTER COLUMN TYPE is simpler
and also much faster for large tables.

Also fixed the inappropriate name used for
temporary columns during column type casting
(which *does* require the copy+rename technique).

bzr revid: odo@openerp.com-20140217164239-0iou3dsiea3foabb