[TAP] sync vs async - time outs

[Gerard]

Hi Tom
> 
> One thing that confuses me a little about this discussion, 
> seems to be the implicit assumption that we don't need to 
> worry about timeouts for asynchronous queries.  
async was included in the discussion 
(http://www.ivoa.net/forum/dal/0903/1028.htm)
> 
> For synchronous services it's a little harder since we need 
> to make sure that we send an appropriate message before the 
> connection dies.  

Indeed, one of the questions I get now and then is what the meaning is of
the message: 

Bad Gateway!
The proxy server received an invalid response from an upstream server. 
...


> Maybe allow a timeout value for synchronous 
> services? [Does it already exist?]
>   While I'm not sure that the underlying databases support 
> timeouts, it's easy enough to put the timeout in the layer 
> that calls the database.  
Postgres' JDBC driver for a long time (still?) did not support the
setQueryTimeout.


Gerard