Re: Reject WAIT FOR earlier in transaction-snapshot mode

> The rationale for this move is to reject unsupported use early so as
> to save some parsing/invalidation effort. The downside of this is that
> it scatters the logic of snapshot management, which is the core of all
> the existing restrictions like running as top-level statements under
> selected isolation levels. That said, we already make the rejection of
> non-top-level statements early in the function, which means they are
> not clustered even for now...

I guess it can be moved, but then instead of what we currently see

```
postgres=# begin isolation level repeatable read ; WAIT FOR LSN
'11110/111111111';
BEGIN
ERROR: invalid input syntax for type pg_lsn: "11110/111111111"
postgres=!#
```

we will get
```
postgres=# begin isolation level repeatable read ; WAIT FOR LSN
'11110/111111111';
BEGIN
ERROR: WAIT FOR cannot be executed within a transaction with an
isolation level higher than READ COMMITTED
postgres=!#
```

It is arguable which one is better here, but I am inclined to prefer
parsing errors first. They tell the user immediately that the supplied
LSN is invalid. Also, the parsing here is very cheap, so I do not think
there is much benefit in trying to reject earlier just to save that work.

Of course, the existing !isTopLevel restriction behaves differently here,
since it fires before we parse the WAIT FOR arguments at all. If I were
inclined to change anything else, I would move both the top-level and
isolation-level checks to just after parsing but before

This way for top-level we get:

```
postgres=# DO $$
BEGIN
EXECUTE 'WAIT FOR LSN ''111111111/11''';
END
$$;
ERROR: invalid input syntax for type pg_lsn: "111111111/11"
CONTEXT: SQL statement "WAIT FOR LSN '111111111/11'"
PL/pgSQL function inline_code_block line 3 at EXECUTE
postgres=#
```
instead of

```
postgres=# DO $$
BEGIN
EXECUTE 'WAIT FOR LSN ''0/0''';
END
$$;
ERROR: WAIT FOR can only be executed as a top-level statement
DETAIL: WAIT FOR cannot be used within a function, procedure, or DO block.
CONTEXT: SQL statement "WAIT FOR LSN '0/0'"
PL/pgSQL function inline_code_block line 3 at EXECUTE
postgres=#
```

but not sure that's worth it either.

--
Sami