Products-zctextindex

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- Fixed problem where the index was not reindexed if the new value was an empty
string leading to inconsistence between the object attribute (that is empty)
and the index that still contains the old indexed value.

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- Fixed problem in reindex document optimization, which could lead to negative
document counts when reindexing unchanged documents.

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- Avoid changing data, if the indexed values stayed the same.

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- Changed word id creation algorithm in Lexicon. Instead of relying on an
increasing length counter, we use a number from a randomized range. This
avoids conflict errors while adding new words in multiple parallel
transactions. Inspired by code from ``enfold.fixes``.

- Lexicon: Added clear method.

- Lexicon: Removed BBB code for instances created with Zope < 2.6.2.

- Added missing namespace_packages declaration to setup.py.

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- Released as separate package.