Finished implementation of session and one-time-key stores for RDBMS backends. Refactored the API of sessions and their interaction with the backend database a fair bit too. Added some session tests. Nothing testing ageing yet, 'cos that's a pain inna ass to test :) Note: metakit backend still uses the *dbm implementation. It might want to implement its own session store some day, as it'll be faster than the *dbm one.

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