pgsql: Modify tuptoaster's API so that it does not try to modify the

Log Message:
-----------
Modify tuptoaster's API so that it does not try to modify the passed
tuple in-place, but instead passes back an all-new tuple structure if
any changes are needed.  This is a much cleaner and more robust solution
for the bug discovered by Alexey Beschiokov; accordingly, revert the
quick hack I installed yesterday.
With this change, HeapTupleData.t_datamcxt is no longer needed; will
remove it in a separate commit in HEAD only.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.200 -> r1.201)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.200&r2=1.201)
        tuptoaster.c (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c.diff?r1=1.53&r2=1.54)
    pgsql/src/backend/executor:
        execMain.c (r1.259 -> r1.260)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.259&r2=1.260)
    pgsql/src/include/access:
        tuptoaster.h (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/tuptoaster.h.diff?r1=1.23&r2=1.24)