Ftw.upgrade

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- Optimize memory footprint after every upgrade step. [jone]
- Reduce memory footprint in SavepointIterator by garbage-collecting connection cache. [jone]
- Set the default savepoint threshold to 1000; make it configurable. [jone]
- Enable savepoint iterator by default. Affects ``self.objects``. [jone]
- Use a SavepointIterator in the WorkflowSecurityUpdater in order not to exceed
memory. [mbaechtold]

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- Also catch AttributeErrors when accessing objects of broken brains. [buchi]

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- The upgrade step methods will remove and skip broken catalog brains. [jone]

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- Fix encoding issues in ASCII terminals. [jone]

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- Fix tempfile authentication when created by different user. [jone]

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- Support using ``bin/upgrade`` by another user than the Zope server
with less strict security checks.[jone]