Live reloading of file session provider

[gebn]

The key this is for this not to interrupt throughput. A nice way to do it might be to drop something in each target's event loop for it to pick up when it's ready; need to ensure it cannot run parallel with GC and shutdown. The implementation of this is linked to mapping targets to http.Handlers.

Low priority as this is not a problem we currently have.

[gebn]

fsnotify also requires some sort of OS rather than a scratch base container image.