nts: rework update of NTP server address

nts: rework update of NTP server address

In the NTS-NTP client instance, maintain a local copy of the NTP address
instead of using a pointer to the NCR's address, which may change at
unexpected times.

Also, change the NNC_CreateInstance() to accept only the NTP port to
make it clear the initial NTP address is the same as the NTS-KE address
and to make it consistent with NNC_ChangeAddress(), which accepts only
one address.