Use duration ctor for asio timer in NIA setAlarm

NetworkInterfaceASIO currently creates timers for setAlarm using an absolute time point rather than a duration. Our tests were switched to use mock timers, and this is resulting in taking a mock timer time point and setting an expiry as though it was a time point on the system clock. Switching setAlarm to use durations relative to NetworkInterfaceASIO::now() will fix this issue in our test.