udp: use slab for pending scrape responses to fix potential issue

Peers sometimes send multiple scrape requests with the same transaction
id, which would previously cause warnings due to replacing the
PendingScrapeMapEntry and later not finding it.

aquatic_udp_bench/src/scrape.rs

@@ -41,8 +41,7 @@ pub fn bench_scrape_handler(
         for request_chunk in requests.chunks(p) {
             for (request, src) in request_chunk {
                 let request = ConnectedRequest::Scrape(PendingScrapeRequest {
-                    connection_id: request.connection_id,
-                    transaction_id: request.transaction_id,
+                    slab_key: 0,
                     info_hashes: request
                         .info_hashes
                         .clone()