pgsql: Disallow committing a prepared transaction unless we are in the

Log Message:
-----------
Disallow committing a prepared transaction unless we are in the same database
it was executed in.  Someday it might be nice to allow cross-DB commits, but
work would be needed in NOTIFY and perhaps other places.  Per Heikki.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        twophase.c (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.27&r2=1.28)