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 don't feel too strongly about this either way, but I should not that most of the tests which have comments follow the normal godoc convention. I always do if I write a comment for tests, and I always do try to write a comment for tests unless they're truly trivial.

I don't like the inconsistency of having multiple conventions for comments at the top of methods.