Refactor 2: make callers responsible for sessions

[msm-cert]

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Second round of refactoring - move session management out of the database object (to make atomicity easier).

Beware of the dragons, this code has a long history of concurrency bugs. In the previous version, there was an implicit commit all the time... I'm not sure there are no places where i should call .commit() directly, but so far it works in my tests.

[msm-cert]

Refactoring can wait for another day. Currently I have no plans of merging this.