Revert patch becaues of locking concerns:

doc/src/sgml/ddl.sgml

doc/src/sgml/ref/alter_index.sgml

doc/src/sgml/ref/alter_table.sgml

src/backend/catalog/pg_constraint.c

src/backend/catalog/pg_depend.c

src/backend/commands/alter.c

src/backend/parser/gram.y

src/backend/tcop/utility.c

src/include/catalog/dependency.h

src/include/catalog/pg_constraint.h

src/test/regress/expected/alter_table.out

src/test/regress/sql/alter_table.sql