Less global variables

Hackers,

In order to limit the number of global variables each transaction needs,
I plan to submit parts of the previous nested transaction patch. Those
are the deferred trigger queue and the snapshot structures.

The idea is that those patches do not change any functionality, and
should be an acceptable intermediate state between the current code and
the nested transactions code, so they can be applied immediately. Also
they help me keep the nested transaction patch smaller so it can be
reviewed more quickly.

Comments?

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"Nunca confiaré en un traidor. Ni siquiera si el traidor lo he creado yo"
(Barón Vladimir Harkonnen)