revamp leap second handling

[davidv1992]

There is an issue where the os interface inserts leap seconds at the end of the month, whilst ntp allows the flag to be set 3 months in advance, with the only valid insertion points being end of december and end of june.

Furthermore, it would be good to support leap second files as an alternative source for leap second information.

Finally, we may want to implement a no-change no-query period around leap seconds to deal with the issue of repeated timestamps (this should not be necessary, but it is hard to test).