log_min_duration_statement feature

OK,

I've implemented the TODO feature:

* Add GUC log_statement_and_duration to print statement and >= min
duration

..except I've given it a more consistent name.

This new feature will really help me find slow queries in my codebase.

The only thing that I'm not 100% on is the ong >= int * 1000 comparison I
do - will that overflow if the user has supplied max int?

Chris