SSI patch version 14

Attached.
 
Dan and I have spent many, many hours desk-check and testing,
including pounding for many hours in DBT-2 at high concurrency
factors on a 16 processor box.  In doing so, we found and fixed a few
issues.  Neither of us is aware of any bugs or deficiencies
remaining, even after a solid day of pounding in DBT-2, other than
the failure to extend any new functionality to hot standby.
 
At Heikki's suggestion I have included a test to throw an error on an
attempt to switch to serializable mode during recovery.  Anything
more to address that issue can be a small follow-up patch -- probably
mainly a few notes in the docs.
 
-Kevin