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. 
Hmm. I understand what you are saying, but if I leave off the make then the static analyzer complains: "expected expression". And if I don't have anything then I have not initialized the map.