Re: Reject WAIT FOR earlier in transaction-snapshot mode

Hi,

Thanks for the review.

> BEGIN ISOLATION LEVEL READ COMMITTED;
> DECLARE c CURSOR FOR SELECT 1;
> WAIT FOR LSN '0/0';
> -- ERROR: WAIT FOR must be called without an active or registered snapshot
>
> BEGIN ISOLATION LEVEL READ COMMITTED;
> SELECT pg_export_snapshot();
> WAIT FOR LSN '0/0';
> -- ERROR: WAIT FOR must be called without an active or registered snapshot

This means there is a bug in the ERROR message DETAIL. Thanks for pointing
this out.

```
postgres=# begin;
BEGIN
postgres=*# DECLARE c CURSOR FOR SELECT 1;
DECLARE CURSOR
postgres=*# WAIT FOR LSN '0/0' WITH (mode 'PRIMARY_FLUSH');
ERROR: WAIT FOR must be called without an active or registered snapshot
DETAIL: WAIT FOR cannot be executed within a transaction with an
isolation level higher than READ COMMITTED.
postgres=!#
```

So I fixed this by first checking the isolation level and using "WAIT
FOR cannot be executed ... READ COMMITTED"
as the errmsg. The existing snapshot check must come after that.

I also added tests to cover the three relevant cases: READ COMMITTED
with a snapshot, and
REPEATABLE READ both with and without a snapshot. The REPEATABLE
READ-with-snapshot
case was already covered, but its expected error message needed to be updated.

The results looks like this:

```
postgres=# BEGIN ISOLATION LEVEL REPEATABLE READ;
BEGIN
postgres=*# select 1;
?column?
----------
1
(1 row)

postgres=*# WAIT FOR LSN '0/0' WITH (mode 'PRIMARY_FLUSH');
ERROR: WAIT FOR cannot be executed within a transaction with an
isolation level higher than READ COMMITTED
postgres=!#
```

```
postgres=# BEGIN ISOLATION LEVEL REPEATABLE READ;
BEGIN
postgres=*# WAIT FOR LSN '0/0' WITH (mode 'PRIMARY_FLUSH');
ERROR: WAIT FOR cannot be executed within a transaction with an
isolation level higher than READ COMMITTED
postgres=!#
```

```
postgres=# BEGIN;
BEGIN
postgres=*# DECLARE c CURSOR FOR SELECT 1;
DECLARE CURSOR
postgres=*#
postgres=*# WAIT FOR LSN '0/0' WITH (mode 'PRIMARY_FLUSH');
ERROR: WAIT FOR must be called without an active or registered snapshot
postgres=!#
```

--
Sami