Disabled for now; when enabled it will allow faster collection
open and close in the normal case, while continuing to downgrade
when exporting or doing a full sync.
Also, when downgrading is disabled, the journal mode is no longer
changed back to delete.
We no longer need to worry about pysqlite implicitly beginning
transactions, and can be more explicit about beginning/ending
transactions
save() now also has a trx argument controlling whether a
transaction should be started / left open