tuptoaster.c must *not* use SnapshotAny

The tuple toaster currently fetches toast-table rows using SnapshotAny.
This is quite uncool, because it will in fact find *any* row.  Including
dead rows resulting from an aborted (or, in 7.2, still-in-progress)
VACUUM.

I believe this is the explanation for a problem report I'm currently
chasing from Joshua Drake:

digivision2=# select * from change_log;
ERROR:  chunk 1 for toast value 3388774 appears multiple times

(there are some thousands of rows with similar problems)

I suggest SnapshotSelf instead.  Comments anyone?
        regards, tom lane