nested transactions

Hackers,

Here is my current patch implementing nested transactions.

At this point I'd like some actual testing. If you have any use for
this please test it and tell me how it behaves for you. Report any
annoyances.

Still missing:
- deal with deferred triggers.
- do something with catcache reference counting

Obvious bugs:
- I just noticed the commit handling of child transactions is wrong. A
concurrent backend could see as committed tuples that should be
regarded as in progress. (Breaks both serializable and read committed
isolation levels.)

subtrans.c should go into src/backend/access/transam/subtrans.c
subtrans.h should go into src/include/access/subtrans.h

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"Investigación es lo que hago cuando no sé lo que estoy haciendo"
(Wernher von Braun)