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 miss Carl](https://cloud.githubusercontent.com/assets/365204/4741068/0cb36c94-5a13-11e4-862e-d7c6a0a822c4.jpg)