Reimplement/optimize doc expiration

[EasyVector]

Expiration was implemented in the CBForest days using only key/value APIs, so it's inefficient. It uses a separate KeyStore with a record for every doc that has an expiration date, and scans through that. What we can do now is add an expiration column to the SQLite table, and use SQL queries to find and purge expired docs. This will be much faster and use less code too.

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