Re: serializable read only deferrable

I wrote: 
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>  
>> I agree that letting it be changed back to read/write after that
>> is surprising and unnecessary.  Perhaps locking down the setting
>> at the time of first grabbing a snapshot would be appropriate. 
>> IIRC that's how it works for transaction isolation level, and
>> this seems like it ought to work the same.
>  
> Agreed.  I can create a patch today to implement this.
 
Attached.
 
Accomplished more through mimicry (based on setting transaction
isolation level) than profound understanding of the code involved;
but it passes all regression tests on both `make check` and `make
installcheck-world`.  This includes a new regression test that an
attempt to change it after a query fails.  I've poked at it with
various ad hoc tests, and it is behaving as expected in those.
 
I wasn't too confident how to word the new failure messages.
 
-Kevin