ntp: Calculate transmit time from receive time and monotonic delta

ntp: Calculate transmit time from receive time and monotonic delta

Calculate transmit time used in NTP as the reception time minus
the delta of the monotonic receive and transmit times. When
calculated this way, it does not matter if the time happens to
be set to something else between the sending and receiving of the
NTP packet.

On sending the added monotonic time and the previous transmit time
are saved at the same point in time. On reception the monotonic time
is evaluated after the packet has been received by ConnMan. This is
in contrast to the actual reception wall clock time which is added
by the kernel. The difference between the reception times on a normal
system is about 100µs, which is neglible.