Model checkpoint overwriting should be smarter

[yawnston]

The model checkpoint file currently contains no information about whether it is the final version. As such, when loading the model from a checkpoint and hot reloading is enabled, the first iteration of hot reloading will overwrite the model checkpoint if overwriting is configured.

It would be nice if the overwriting was smarter:

• If there existed no model checkpoint at startup, overwriting should be enabled for every iteration of hot reloading.
• If there existed a model that was not final, overwriting should be enabled for every iteration of hot reloading.
• If there existed a model that was final, either hot reloading should not run at all, or it should only overwrite the model checkpoint file on the last iteration.