Transactions

[RealOrangeOne]

The current backends don't explicitly interact with transactions. DummyBackend and ImmediateBackend don't touch it at all, and DatabaseBackend can introduce race conditions if not running in a transaction, as the task may begin execution before the data is fully saved.

There are lots of comments on the DEP (example) about this and understanding the semantics.

This will need working out, as there's a difference between how the built-in backends function, which could lead to odd bugs in production which unit tests wouldn't pick up.

[RealOrangeOne]

Implemented in #57