we have an easy way to discover such listeners when adding new file systems.

Avoid calling directly subscribe/unsubscribe from the FsChangeListener API
and rely on the FileSystem to do this so that we properly populate the
interests of the FsChangeListener while at the same time populating the
map of listeners and the keys that they are interested in for the current
file system.

This double registering mechanims is needed to have FsChangeListeners know
to what channles (file system queue paths) and keys they are interested in
and also for the FileSystem objects to know the same information so that
they can properly notify the FsChangeListeners when there is an update that
might interest them.