Zc-zodbdgc

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- The previous version deleted too many objects at a time, which could
put too much load on a heavily loaded storage server.

- Add a sleep or allow the storage to rest after a set of deletions.
Sleep for twice the time taken to perform the deletions.

- Adjust the deletion batch size to take about .5 seconds per
batch of deletions, but do at least 10 at a time.

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- Optimized garbage collection by using a temporary file to
store object references rather than loading them from the analysis
database when needed.

- Added an -f option to specify file-storage files directly. It is
wildly faster to iterate over a file storage than over a ZEO
connection. Using this option uses a file iterator rather than
opening a file storage in read-only mode, which avoids scanning the
database to build an index and avoids the memory cost of a
file-storage index.

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- Added an option to ignore references to some databases.

- Fixed a bug in handling of the logging level option.

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Initial release