Add support for LIMIT clause in GROUP_CONCAT()

GROUP_CONCAT supports ORDER BY and DISTINCT but not LIMIT clause.
Supporting LIMIT N[,M] could simplify queries.

something like

SUBSTRING_INDEX(GROUP_CONCAT(CONCAT_WS(":",date,cnt) ORDER BY cnt DESC),",",1)

could be written as

GROUP_CONCAT(CONCAT_WS(":",date,cnt) ORDER BY cnt DESC LIMIT 1)

this can be used in timeseries-like calculations. Suppose we have a table (date,cnt) and we want to produce a list of dates, for each year, where cnt was maximum for that year, showing such date,cnt for each year

currently this is written in an awkward manner and is also quite inefficient as we SELECT much more data that is necessary for the task:

SELECT LEFT(col1,4) AS _yyyy
, SUBSTRING_INDEX(GROUP_CONCAT(CONCAT_WS(":",DATE_FORMAT(col1,"%a %Y-%m-%d"),LPAD(col2,8," ")) ORDER BY col2 DESC),",",1) AS col3
FROM t1
GROUP BY 1
;