Fix slowdown of server/ unittests and prevent possible flaky test
Merge pull request #141 from embark/embark/fix_slow_server_tests

Fix slowdown of server/ unittests and prevent possible flaky test
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 makes it more likely to get a good measurement before we
determine the offset interval.
Address PR comments