ntp: limit number of interleaved responses in symmetric mode

In symmetric mode, don't send a packet in interleaved mode unless it is
the first response to the last valid request received from the peer and
there was just one response to the previous valid request. This prevents
the peer from matching the transmit timestamp with an older response if
it can't detect missed responses.

test/simulation/122-xleave

@@ -17,6 +17,7 @@ max_sync_time=500
 base_delay="(+ 1e-4 (* -1 (equal 0.1 from 2) (equal 0.1 to 1)))"
 
 client_lpeer_options="xleave minpoll 5 maxpoll 5"
+client_rpeer_options="minpoll 5 maxpoll 5"
 
 run_test || test_fail
 check_chronyd_exit || test_fail