xact.c cleanup -- uncontroversial try

Hackers,

This is a cleanup patch for access/transam/xact.c.  It is a trimmed down
version of the previous patch, with the controversial part removed.  It
only removes some #ifdef NOT_USED code, and adds a new TBLOCK state
which signals the fact that StartTransaction() has been executed.

Please review and apply if OK.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"El sabio habla porque tiene algo que decir;
el tonto, porque tiene que decir algo" (Platon).