Ignore read-only transaction cache when getting a category from a new or temp object

The purpose is to be able to use the amount generator during indexation.
At some point, it executes:

        if amount.getQuantityUnit():
        ...
        for x in property_dict.iteritems():
          amount._setProperty(*x)

where `amount.getQuantityUnit()` may getResource before it is set.
Any further access to the resource category would be wrong.

There may be a way to only change the amount generator but this kind
of pitfall is likely to happen in many other places if we keep such
a read-only transaction cache for new or temp objects.

See merge request nexedi/erp5!1818

product/CMFCategory/CategoryTool.py

@@ -840,6 +840,9 @@ class CategoryTool(BaseTool):
       # XXX: This cache is rarely useful, and the overhead quite important.
       #      It would certainly become counter-productive if any significative
       #      improvement was done to the cached methods.
+      if getattr(context, '_p_jar', None) is None: # new or temporary
+        cache = None
+      else:
         cache = getReadOnlyTransactionCache()
         if cache is not None:
           key = ('getSingleCategoryAcquiredMembershipList', context,