Universal cancellation support

Technically, you're supposed to be allowed to cancel _any_ request with `$/cancel`. We don't support this at the moment.

To do this we would need to run every handler in a thread so we can kill it. So this is very related to https://github.com/haskell/lsp/issues/409

Start by reading the cancellation requirement for `$/cancel` and the related issue #409. Examine how request handlers are currently run and determine the design needed to cancel every handler safely. Done means any request can be cancelled through `$/cancel` without leaving the handler running.

Written by the indexing model from the issue text.