Improvement for high availability

I would like to propose following solution which resolve problem with crashes of `ntp-server` or defined by user `external-ntp-servers`. 

This fix lets to `ntp-server` crash or network problems without affecting clock stability in the system. Because internal clock is fudged to have stratum 16, which is really low, it  has always the lowest priority. If `ntp-server` will be available clock will synchronized with them.

I've tested many different expanded configurations, but this patch seems to be the most reasonable solutions.