GUC changes to log query durations

The attached patch adds 'log_duration' to GUC, showing the duration of
each query. It is to be used with LOG_PID and LOG_STATEMENT:

[22602]: LOG: duration: 0.079464 sec
[22602]: LOG: duration: 0.079464 sec
[22602]: LOG: duration: 0.079464 sec
[22602]: LOG: duration: 0.079464 sec

The patch also changes debug_print_query to log_statement and
show_query_statistics to show_statement_statistics. I believe Peter has
suggested similar changes recently, making the debug_print_query a log_
option because it really isn't for debugging the server, and changing
'query' to 'statement' to be more precise.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
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