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: query: SELECT usesuper FROM pg_catalog.pg_user WHERE usename = 'postgres'
[22602] LOG: duration: 0.666637 sec
[22602] LOG: query: SELECT * FROM pg_language;
[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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