bugfix: fix wrong partial Response sender access

- replaces improper record field access of `sender`, only existing in a
  Request, by `senderID` of a Response
- fixes the resulting exception-crash
- adds new function that enqueues a verification mark and timestamp bump
  of an existing cache entry

src/Hash2Pub/DHTProtocol.hs

@@ -658,8 +658,7 @@ requestStabilise ns neighbour = do
                                                       )
                                                       ([],[]) respSet
             -- update successfully responded neighbour in cache
-            now <- getPOSIXTime
-            maybe (pure ()) (\p -> queueAddEntries (Identity $ RemoteCacheEntry (sender p) now) ns) $ headMay (Set.elems respSet)
+            maybe (pure ()) (\p -> queueUpdateVerifieds (Identity $ senderID p) ns) $ headMay (Set.elems respSet)
             pure $ if null responsePreds && null responseSuccs
                       then Left "no neighbours returned"
                       else Right (responsePreds, responseSuccs)
@@ -826,6 +825,18 @@ queueDeleteEntry :: NodeID
                  -> IO ()
 queueDeleteEntry toDelete = queueDeleteEntries $ Identity toDelete
 
+
+-- | enqueue the timestamp update and verification marking of an entry in the
+-- global 'NodeCache'.
+queueUpdateVerifieds :: Foldable c
+                     => c NodeID
+                     -> LocalNodeState s
+                     -> IO ()
+queueUpdateVerifieds nIds ns = do
+    now <- getPOSIXTime
+    forM_ nIds $ \nid' -> atomically $ writeTQueue (cacheWriteQueue ns) $
+        markCacheEntryAsVerified (Just now) nid'
+
 -- | retry an IO action at most *i* times until it delivers a result
 attempts :: Int             -- ^ number of retries *i*
          -> IO (Maybe a)    -- ^ action to retry