Handle cancellation requests in LSP #75
Description
The LSP supports cancellation requests to "cancel" requested information that the client no longer needs, e.g. because the document was closed or changed since the request was sent.
We should add support for cancellation requests. One way of implementing this is to keep a map of pending-requests (keyed by request id) and skipping requests if their id isn't in the map. Cancelling a request is than as simple as removing the id from said map (which we also need to do when completing requests).