remove lingering references to closed sockets (#219)

Author: zarqman

lib/dnsruby/select_thread.rb

@@ -393,6 +393,9 @@ def remove_id_from_mutex_synchronized_block(id)
       if !persistent?(socket) || max_attained?(socket)
         @@sockets.delete(socket)
         @@socket_hash.delete(socket)
+        @@socket_remaining_queries.delete(socket)
+        @@socket_is_persistent.delete(socket)
+        @@tcp_buffers.delete(socket)
         Dnsruby.log.debug("Closing socket #{socket}")
         socket.close rescue nil
       end