Deobfuscate scalems.store.FileStore lifetime management.

[eirrgang]

scalems.store makes "clever" use of the Python generator protocol to place the main owning reference to the FileStore in the frame of a generator function.

This is done for various reasons that seem insufficient justification for the obscurity of the implementation. (Namely, a generator instance seemed like kind of a handy relocatable context manager that allowed for a finally block.)

We can probably improve readability, maintainability, and simplicity by using a __del__ method or a finalizer callback (ref: the weakref module).

This becomes more relevant as the FileStoreManager gains responsibilities beyond managing that single generator.

There may be other reasons for the original design decision, though, and I don't want to open a fresh can of worms just yet, so this issue is deferred until after the "raptor migration" iteration