feat(loader): add reload strategy, rewamp hot_reload

[joelwurtz]

This replace the hot_reload with a strategy that also allows to always generate the mappers even if no change.

This is useful for us when we change something that could not be detected, this will also defnitely be useful for users in debug env when we will collect metadata for generator, this will avoid collecting only on the first "request" and avoid confusion.

Never strategy is the default one in non debug for further optimization.