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. 
This is the convention for godoc, and I like to follow it for unexported methods too, but I don't think it's necessary for tests. Test functions tend to have very wordy names, and repeating those names in the comment doesn't really improve clarity.