SVN-1.5 FSFS scalability updates

[Dan Christian]

Hey, I'm new to contributing to the the book.

There have been some updates to FSFS scalability that probably should
be added to the section in Chapter 5 on FSFS.  These are in the
release notes, but I don't know if that means they automatically get
pulled into the book.

I also have some notes on high availability, high performance
deployments that could go in.

The note on FSFS being immature should probably be adjusted.  It
received quite a lot of testing during 1.4 deployment and 1.5
development.

-Dan C