[GeneralDiscussion] performance fixes

I looked at some more server slowness today, with the help of emacs,  
dtach, top, vmstat, tail, highlight-lines-matching-regexp, and Tres  
Seaver's excellent thrashcatcher product which shows zodb load and  
store counts for each transaction in the trace log. I applied two  
fixes which large zwiki sites might want to know about:

- bots were finding their way directly to page revision diffs (how ? I  
don't know), trying every link and causing a lot of unnecessary zodb  
loads. I stopped it by adding this apache rewrite rule::

    # temporary to keep bots out of revisions
    # redirects direct requests for revisions to the latest page
    RewriteRule (.*)/revisions/(.*)\.[0-9]+/ $1/$2 [R,L]

- visits to old revisions of zwiki.org FrontPage were generating  
thousands of page loads every time. The "new pages" dtml snippet on  
FrontPage calls pages() to show the last 5 pages, but in the revisions  
folder this was falling back to a non-catalog search (since there is  
no catalog there), loading all 2000+ old page revisions into cache.  
For this reason pages now returns nothing without a properly  
configured catalog.

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forwarded from http://zwiki.org/GeneralDiscussion#[email protected]