Remove stratum from the NTP sample and update it together with the leap
status. This enables a faster update when samples are dropped by the NTP
filters.
Remove leap status from the NTP sample and set it independently from
the sample accumulation in order to accept a leap second sooner when
samples are filtered.
Move the implementation of the median filter to a separate file to make
it useful for NTP. Replace some constants with parameters and generalize
the code to work with full NTP samples (including root dispersion/delay,
stratum, and leap).
For refclocks it should give the same results as before.
