Currently if you provide an ANN search using the `knn` search section, the search doesn't respect cancellation. This is because ANN searches use their own Lucene data structures that aren't covered by `ExitableDirectoryReader`.

This class is forked from Lucene, and in Lucene's version we recently added fine-grained cancellation for ANN search (https://github.com/apache/lucene/pull/833). So I think we just need to fold in the changes to the Elasticsearch version.
server/src/main/java/org/elasticsearch/search/internal/ExitableDirectoryReader.java
private final PointValues in;
        private final QueryCancellation queryCancellation;
        private ExitablePointValues(PointValues in, QueryCancellation queryCancellation) {
            this.in = in;
            this.queryCancellation = queryCancellation;
            this.queryCancellation.checkCancelled();
        }
        @Override
        public PointTree getPointTree() throws IOException {
            queryCancellation.checkCancelled();
            return new ExitablePointTree(in, in.getPointTree(), queryCancellation);
        }
        @Override
        public byte[] getMinPackedValue() throws IOException {
            queryCancellation.checkCancelled();
            return in.getMinPackedValue();
        }
        @Override
        public byte[] getMaxPackedValue() throws IOException {