GraphSync can partially cancel a request

This isn't necessary for the MVP but GraphSync will _eventually_ need some way to partially cancel a request. We can do this by either:

1. Adding a way to tell a remote peer to _not_ send some sub-dag rooted at a specific CID. This will require a protocol change but may be the easiest to implement.
2. Making it possible to atomically cancel one selector and replace it with another _without_ sending duplicate blocks. To do this:
a. The requester would need to be able to create a selector for the remainder of an in-progress selector.
b. The responder would need to keep track of the last N blocks (or just a bloom filter?) sent to the requester as the replacement selector will end up overlapping with the original selector.