Add COPY performance summary log message

[pingles]

Copy statement performance is stored in the stl_s3client table. It'd be awesome to see a log message at the end of the COPY statement to show how much data was loaded, how long it took, the size of the transfer etc.

Example query below:

SELECT SUBSTRING(q.querytxt,1,60) AS querytxt,
       s.n_files,
       transfer_size_mb,
       data_size_mb,
       s.time_seconds,
       s.transfer_size_mb/decode(s.time_seconds,0,1,s.time_seconds) AS transfer_mb_per_s,
       s.data_size_mb/decode(s.time_seconds,0,1,s.time_seconds) AS data_mb_per_s
FROM
(
  SELECT query,
         COUNT(*) AS n_files,
         SUM(transfer_size/(1024*1024)) AS transfer_size_MB,
         SUM(data_size/(1024*1024)) AS data_size_MB,
         (MAX(end_Time) - MIN(start_Time))/(1000000) AS time_seconds,
         MAX(end_time) AS end_time
  FROM   stl_s3client
  WHERE  query > 0 AND transfer_time > 0
  GROUP BY query
) AS s
LEFT JOIN stl_Query AS q ON q.query = s.query
ORDER BY end_time DESC
LIMIT 10;