ntp: wait for late HW TX timestamps

When sending client requests to a close and fast server, it is possible
that a response will be received before the HW transmit timestamp of
the request itself. To avoid processing of the response without the HW
timestamp, monitor events returned by select() and suspend reading of
packets from the receive queue for up to 200 microseconds. As the
requests are normally separated by at least 200 milliseconds, it is
sufficient to monitor and suspend one socket at a time.

ntp_io.c

@@ -318,6 +318,9 @@ close_socket(int sock_fd)
   if (sock_fd == INVALID_SOCK_FD)
     return;
 
+#ifdef HAVE_LINUX_TIMESTAMPING
+  NIO_Linux_NotifySocketClosing(sock_fd);
+#endif
   SCH_RemoveFileHandler(sock_fd);
   close(sock_fd);
 }
@@ -685,6 +688,11 @@ read_from_socket(int sock_fd, int event, void *anything)
   unsigned int i, n;
   int status, flags = 0;
 
+#ifdef HAVE_LINUX_TIMESTAMPING
+  if (NIO_Linux_ProcessEvent(sock_fd, event))
+    return;
+#endif
+
   hdr = ARR_GetElements(recv_headers);
   n = ARR_GetSize(recv_headers);
   assert(n >= 1);