rep_security merge to trunk

[Damien Katz]

I think the rep_security branch is looking pretty solid. I still have  
work to do to merge with Adam's recent replicator changes.

This patch breaks the file format and replication API, so replication  
with earlier versions is not possible. And the "all or nothing w/  
conflict checking" transactions are gone. Which I think is good,  
because people were relying on it without understanding the rest of  
CouchDB doesn't support that feature.

I'd like to go ahead and merge this to trunk. Comments, suggestions  
and objections please.

-Damien