pgsql: Fix toast table creation.

Fix toast table creation.

Instead of using slightly-too-clever heuristics to decide when we must
create a TOAST table, just check whether one is needed every time the
table is altered.  Checking whether a toast table is needed is cheap
enough that we needn't worry about doing it on every ALTER TABLE command,
and the previous coding is apparently prone to accidental breakage:
commit 04e17bae50a73af524731fa11210d5c3f7d8e1f9 broken ALTER TABLE ..
SET STORAGE, which moved some actions from AT_PASS_COL_ATTRS to
AT_PASS_MISC, and commit 6c5723998594dffa5d47c3cf8c96ccf89c033aae broke
ALTER TABLE .. ADD COLUMN by changing the way that adding columns
recurses into child tables.

Noah Misch, with one comment change by me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/39a68e5c6ca7b41b889e773ca58535324af69630

Modified Files
--------------
src/backend/catalog/toasting.c            |   10 +++++-----
src/backend/commands/tablecmds.c          |   10 ++--------
src/test/regress/expected/alter_table.out |   13 +++++++++++++
src/test/regress/input/misc.source        |    6 ++++++
src/test/regress/output/misc.source       |   11 +++++++++++
src/test/regress/sql/alter_table.sql      |   10 ++++++++++
6 files changed, 47 insertions(+), 13 deletions(-)