ntp: separate timestamps for restarting symmetric protocol

ntp: separate timestamps for restarting symmetric protocol

Save the local receive and remote transmit timestamp needed for
(re)starting the symmetric protocol when no valid reply was received
separately from the timestamps that are used for synchronization of the
local clock.

This extends the interval in which the local NTP state is (partially)
protected against replay attacks in order to complete a measurement
in the interleaved symmetric mode from [last valid RX, next TX] to
[last TX, next TX], i.e. it should be the same as in the basic mode.