SQLAlchemy store should remove finished schedules on cleanup

[Reskov]

It looks like a typo error because a similar code in the mongodb/memory module deletes finished schedules.

I have modified a test case for two reasons:

1. it was not executed on all data stores
2. the anyio.Event() is not serializable across different datastores, resulting in an error before execution.

[coveralls]

coverage: 92.596% (+0.04%) from 92.561%
when pulling c941712 on Reskov:asyncpg-cleanup-fix
into 65b7606 on agronholm:master.

[agronholm]

Looks good at a glance. I'll give it a proper review within a couple days.

[agronholm]

Thank you!