pgsql: Decouple the values of TOAST_TUPLE_THRESHOLD and

Log Message:
-----------
Decouple the values of TOAST_TUPLE_THRESHOLD and TOAST_MAX_CHUNK_SIZE.
Add the latter to the values checked in pg_control, since it can't be changed
without invalidating toast table content. This commit in itself shouldn't
change any behavior, but it lays some necessary groundwork for experimentation
with these toast-control numbers.

Note: while TOAST_TUPLE_THRESHOLD can now be changed without initdb, some
thought still needs to be given to needs_toast_table() in toasting.c before
unleashing random changes.

Modified Files:
--------------
pgsql/doc/src/sgml:
storage.sgml (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml.diff?r1=1.15&r2=1.16)
pgsql/src/backend/access/heap:
heapam.c (r1.230 -> r1.231)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.230&r2=1.231)
tuptoaster.c (r1.72 -> r1.73)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c.diff?r1=1.72&r2=1.73)
pgsql/src/backend/access/transam:
xlog.c (r1.265 -> r1.266)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.265&r2=1.266)
pgsql/src/bin/pg_controldata:
pg_controldata.c (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_controldata/pg_controldata.c.diff?r1=1.34&r2=1.35)
pgsql/src/bin/pg_resetxlog:
pg_resetxlog.c (r1.58 -> r1.59)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_resetxlog/pg_resetxlog.c.diff?r1=1.58&r2=1.59)
pgsql/src/include/access:
tuptoaster.h (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/tuptoaster.h.diff?r1=1.33&r2=1.34)
pgsql/src/include/catalog:
pg_control.h (r1.36 -> r1.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_control.h.diff?r1=1.36&r2=1.37)