ntp: don't replace source instance when changing address

Add new functions to change source's reference ID/address and reset the
instance. Use that instead of destroying and creating a new instance
when the NTP address is changed.
sources: allow setting minsamples and maxsamples for each source

The minsamples and maxsamples directives now set the default value,
which can be overriden for individual sources in the server/peer/pool
and refclock directives.