The comment about handles passed by our caller no longer applies, remove it.

Don't bother clearing session.dhandle before jumping to err, err makes the
same change.
remove unnecessary WT_UNUSED()
Add a compaction option, set compaction 10% of the time.
Switch first-fit allocation set from 20% to 10% of the time.
Always seed the random number generator, otherwise the first time format
runs is always the same run (and if you wrap runs in a loop, you get the
same run over and over).