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Implement a worker process pool for running many, short-lived jobs
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Restore pre-refactor save to disk performance by running copy and metadata update in parallel
Review downloaded metadata: Add suggestions for the tags, authors, publishers and series fields. Suggestions will pop-up as you type. Fixes #1390887 [Tags suggestions in review downloaded metadata dialog](https://bugs.launchpad.net/calibre/+bug/1390887)
Remove unused code
Linux: Fix regression on Unity that prevented some keyboard shortcuts from working because of the global menu bar. Fixes #1390901 [Book Merge via Keyboard Shortcut Not Working](https://bugs.launchpad.net/calibre/+bug/1390901)
Add a facility to send common data (common to all jobs) to the workers in a pool
Send the plugboard cache only once per worker instead of once per job
Method that allows implementing has_book in a worker process with no access to the data
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A spot of refactoring
Small optimization, only serialize common_data once instead of once per worker
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Method to find identical books in worker processes without access to the db