Those are single-valued, so just check consistency internally and return
a single value.
Changelog and some cleanups by Vincent Pelletier <vincent@nexedi.com>.

It is known to cause table locks when no value is provided for such
column. We now always generate uids in ERP5, so this setting is useless.

-user with correct permissions should be able to set start or stop date. Add proxy role manager in scripts.

See testTradeModelLine for new use of taxes/discounts.

This avoid random permission errors (like if another user run the
same test)

This is a followup to commit 8995b645
("always start transactions with begin() and make tic() commit automatically"):
there is now no need to use self.stepTic() instead of self.tic()

Select methods do not need to check if catalog data is up-to-date:
this is always done automatically by searchMovementList()

Update 'rafael' branch to use new changes on unit tests and check that the last merge did not add random failures on functional tests

Conflicts:
	product/ERP5Catalog/CatalogTool.py

Add generic InteractionGadget implementation.

Use of meta types is deprecated. Ignoring them makes it possible to optimize
getTypeBasedMethod thanks to portal types as classes.

- New readOnlyTransactionCache context manager replacing
  enableReadOnlyTransactionCache and disableReadOnlyTransactionCache
- Drop compatibility code that tolerate a dummy context to be passed to
  getReadOnlyTransactionCache and getTransactionalVariable.