Begin work on LocalDiskStorageController in the `puterfs` extension.
This replaces LocalDiskStorageStrategy and LocalDiskStorageService from
the core. The `upload()` method is implemented to verify that it's
working.
This commit by itself will break other storage strategies. The next step
is to allow extensions to register storage controllers for puterfs. Part
of that work is done in this commit by emitting an event to register
storage controllers, but this commit does not include a way to
configure/select storage controllers.
