Use correct clock when processing RR with lsr older than our last SR

The NTP timestamp in this library's SR messages is generated from the RTC (rather than monotonic), as expected. When a RR is received that has lsr not equal to the last SR we sent, the local clock is used to enable a roughly-accurate comparison between the LSR and now. Previously comparison was using the monotonic clock which can have a significantly different value for now than the RTC, resulting in incorrect calculated values for RTT. This patch corrects the RR handler to use the RTC clock so that comparisons are in the same range and therefore RTT values are calculated correctly.