Pq

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- Fix queue class factory pattern.

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- Fix compatibility with `NamedTupleCursor`.

- Fix duplicate column name issue.

- Add option to specify own queue class.

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- Fixed Python 2 compatibility.

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- Added worker class and handler helper decorator.
[jeanphix]

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- Python 3 compatibility.
[migurski]

- Fix time zone issue.

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Improvements:

- Fixed concurrency issue where a large number of locks would be held
as a queue grows in size.

- Fixed a database connection resource issue.