pgsql: Add an extra header byte to TOAST-pointer datums to represent

Log Message:
-----------
Add an extra header byte to TOAST-pointer datums to represent their size
explicitly.  This means a TOAST pointer takes 18 bytes instead of 17 --- still
smaller than in 8.2 --- which seems a good tradeoff to ensure we won't have
painted ourselves into a corner if we want to support multiple types of TOAST
pointer later on.  Per discussion with Greg Stark.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        storage.sgml (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?r1=1.19&r2=1.20)
    pgsql/src/backend/access/heap:
        tuptoaster.c (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c?r1=1.75&r2=1.76)
    pgsql/src/include/catalog:
        catversion.h (r1.429 -> r1.430)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.429&r2=1.430)
    pgsql/src/include:
        postgres.h (r1.83 -> r1.84)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postgres.h?r1=1.83&r2=1.84)