Item12950: Use a different log rotate strategy

Copying the file contents just takes too long.  Rename the file.

Also removed the code for flock unlock.  It's recommended to let the
close do the unlock, so that any buffers written during the close don't
happen after the lock has been released.

git-svn-id: http://svn.foswiki.org/trunk/core@17792 0b4bb1d4-4e5a-0410-9cc4-b2b747904278