Switch to using a SQLite3 database for spambayes classifier training data, instead of a pickle file.
Author: exarkun
Reviewer: mithrandi
The reasons for preferring SQLite3 are manifold. Performance, reliability, inspectability,
and maintainability should all be improved by this change. An upgrader is included, of
course, to convert old data. However, the old pickle file is left in place for manual
deletion afterwards, at the discretion of a system administrator.