Sample the log traffic to work out its data rate so that we

Sample the log traffic to work out its data rate so that we
have a better idea of predicting when it will get to the end
of the log. This way we can handle a change in log traffic in
the future. A test to keep an eye on log traffic is more of
a performance test than should be done elsewhere.
Merge of master-melb:xfs-cmds:31053a by kenmcd.

  Sample the log traffic to work out its data rate so that we
  have a better idea of predicting when it will get to the end
  of the log. This way we can handle a change in log traffic in
  the future.