statement_timeout logging

Currently, when we set a statement_timeout and a query runs over that
time there is no log message to say that the statement has timed out. We
do get a message which says
    ERROR:  canceling query due to user request
and so in the server log it is impossible to tell the difference between
a query that has been cancelled deliberately by the user and a query
whose time(out) has come.

The timeout causes the query to be cancelled, which is an ERROR, so it
is possible that the query will be logged if log_min_error_statement is
set to ERROR or below. Again, there is no way to tell the difference
between an ERROR statement that has occurred during execution and an
ERROR statement that is generated by a syntax error. So, if you do set
log_min_error_statement=ERROR then you get swamped by syntax errors
rather than being able to see statement timeouts.

Logging these queries ought to be a valuable source of tuning
information, but as a result of the above, this is not currently the
case.

For now, I've written a quick patch to the log the query text with a
suitable error message.

I expect some debate as to whether the line
           if (log_min_error_statement > WARNING)
should be there at all, so comments are sought.

---

Longer term, we might handle this differently. If we differentiate
between two types of ERROR, at least for log_min_error_statement...
- one that prevents the query from ever running (syntax, parse errors)
- one that occurs because the query itself gets into difficulty, but not
so badly that it constitutes a FATAL error. Lets call it RUNERROR for
now.
I expect most people to want to log RUNERRORs, but to want to ignore the
more normal kind of ERRORs.

When/if we can differentiate between levels, I'll reverse out this patch
since we'll be able to handle it simply with the new errorlevel.

Best Regards, Simon Riggs