TRACKING: WNFS event listeners

[bgins]

This issue tracks changes to be made to implement event listeners in WNFS. This feature was initially requested in #136.

As a quick summary, it would be helpful to provide an interface where users can subscribe to changes when:

• A change is made anywhere in the filesystem
• A specific file or directory changes

In both cases, it would be useful to know when a change occurs locally and on a remote node.

Tasks

• [x] Add an interface to subscribe to root CID changes (change anywhere in the filesystem)
• [x] Add an interface to subscribe to file and directory changes
• [ ] Events for remote changes

[jeffgca]

Note: remote changes are blocked on the server project.