This PR fixes a regression where elaboration of a previous document
version is not cancelled on changes to the document.
Done by removing the default from `SnapshotTask.cancelTk?` and
consistently passing the current thread's token for synchronous
elaboration steps.
This PR fixes an `Elab.async` regression where elaboration tasks are
cancelled on document edit even though their result may be reused in the
new document version, reporting an incomplete result.
While this PR fixes the functional regression, it does so as an
over-approximation by never cancelling such tasks. A follow-up PR will
implement the correct behavior of only cancelling the tasks that are not
reused.
