Last clock offset cleanup.

Limit of 1.5 seconds for measuring clock offset,
to limit the maximum error on a read.

The remote clock reading with smallest error since the last cluster
offset measurement is now used, rather than the latest reading. 
I'm unclear what prompted this change...could you elaborate in the comment?

Also, does it make sense for this to be:

else if oldOffset.MeasuredAt < r.lastMonitoredAt && offset.MeasuredAt > oldOffset.MeasuredAt?