Stop passing the log file handle

The log function was supposed to make users life easier, but constantly
passing the log file handle did not seem to reach this goal. Using a
class wide variable instead.
Add two more stats the pre-log

Collect two kinds of ways to look at the status of the zpools. Note
that this does capture information about more pools than just the ones
under investigation, but that seems reasonable.
Refactor stat writing to print_stats_to_file

Since the pre and post stats are the same,place them in their own
method, call it from both pre and post.