Add a line to the task log whenever task changes status. This only applies
to directly-changed tasks, and not to mass-updates (like all tasks going
from 'completed' to 'queued' on a job requeue).
Be more selective in what's saved to the database to speed some things up.
Most importantly, this avoids saving the entire job when a task status is
updated or a task is assigned.
Requeueing the tasks of a specific worker is now done in the
`TaskStateMachine`, such that it can be called from other services as
well in future commits.
This also makes the `LogStorage` service a dependency of the
`TaskStateMachine`, as it needs to write "this task was requeued" kind
of messages to the task logs.
