pgsql: Adjust CREATE TRIGGER and ALTER TABLE ...

Log Message:
-----------
Adjust CREATE TRIGGER and ALTER TABLE ... ADD FOREIGN KEY to acquire
ExclusiveLock rather than AccessExclusiveLock. This will allow concurrent
SELECT queries to proceed on the table. Per discussion with Andrew at
SuperNews.

Modified Files:
--------------
pgsql/doc/src/sgml:
mvcc.sgml (r2.47 -> r2.48)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml.diff?r1=2.47&r2=2.48)
pgsql/src/backend/commands:
tablecmds.c (r1.148 -> r1.149)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.148&r2=1.149)
trigger.c (r1.178 -> r1.179)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c.diff?r1=1.178&r2=1.179)