[batch] Create pool scheduling loops after loading existing instances

[daniel-goldstein]

Currently, tasks to schedule new instances are put on the event loop inside the Pool and JobPrivateInstanceManager constructors. Pool.create and JobPrivateInstanceManager.create first instantiate an object of their respective type and then load existing instances from the database into the in-memory instance collection. This could potentially cause the create instances loop to trigger while we're drawing "existing" instances, which causes the assertion error in https://github.com/hail-is/hail-tasks/issues/24 when the create instances loop and load instances query race to add the instance to the in-memory data structure.

This change moves the task creation from the constructor to the create method, so we don't start creating instances until all existing instances are accounted for. I think I would have liked to simply pass the constructor a list of instances, but we can't create an Instance without an InstanceCollection.

Resolves hail-is/hail-tasks#24

I also threw in a bit of cleanup, i.e. removing some variable assignments that didn't seem very helpful and resolving a lint issue where we used items where we could just use values.

[daniel-goldstein]

@danking bump

[danking]

love it