ntp: don't use first response in interleaved mode

ntp: don't use first response in interleaved mode

With the first interleaved response coming after a basic response the
client is forced to select the four timestamps covering most of the last
polling interval, which makes measured delay very sensitive to the
frequency offset between server and client. To avoid corrupting the
minimum delay held in sourcestats (which can cause testC failures),
reject the first interleaved response in the client/server mode as
failing the test A.

This does not change anything for the symmetric mode, where both sets of
the four timestamps generally cover a significant part of the polling
interval.