YCQL -- Disallow GROUP BY syntax

[yugabyte-ci]

Jira Link: DB-3446

We don't support the GROUP BY clause in YCQL (only in YSQL) but it is allowed in the syntax (presumably by mistake).

I think the issue is that the group_clause was not disallowed in the parser:
https://github.com/yugabyte/yugabyte-db/blob/master/src/yb/yql/cql/ql/parser/parser_gram.y#L2519 (see for instance the PARSER_UNSUPPORTED calls for some of the other sub-cases below -- the should be there for the top-level non-empty case).

In terms of functionality, from this code: https://github.com/yugabyte/yugabyte-db/blob/master/src/yb/yql/cql/ql/parser/parser_gram.y#L2178 it looks like we are just ignoring the GROUP BY clause if set.

Finally, if backwards compatibility is an issue we can make this a warning rather than an error by default (for GROUP BY specifically.

[skahler-yuga]

A simple test case this would be highlighted here

https://howtoprogram.xyz/2017/02/18/using-group-apache-cassandara/

CREATE TABLE temperature_by_day (
            weatherstation_id text,
            date text,
            event_time timestamp,
            temperature float,
            PRIMARY KEY ((weatherstation_id,date),event_time)
         );
INSERT INTO temperature_by_day(weatherstation_id,date,event_time,temperature)
         VALUES ('1234WXYZ','2016-04-03','2016-04-03 07:01:00',73);

INSERT INTO temperature_by_day(weatherstation_id,date,event_time,temperature)
         VALUES ('1234WXYZ','2016-04-03','2016-04-03 07:02:00',70);

INSERT INTO temperature_by_day(weatherstation_id,date,event_time,temperature)
         VALUES ('1234WXYZ','2016-04-04','2016-04-04 07:01:00',73);
INSERT INTO temperature_by_day(weatherstation_id,date,event_time,temperature)
         VALUES ('1234WXYZ','2016-04-04','2016-04-04 07:02:00',74);

SELECT weatherstation_id, date, MAX(temperature) FROM temperature_by_day GROUP BY weatherstation_id, date;
SyntaxException: Invalid CQL Statement. Selecting aggregate together with rows of non-aggregate values is not allowed
SELECT weatherstation_id, date, MAX(temperature) FROM temperature_by_day
       ^^^^^^^^^^^^^^^^^
                                 GROUP BY weatherstation_id, date;
 (ql error -12)


SELECT MAX(temperature) FROM temperature_by_day GROUP BY weatherstation_id, date;

system.max(temperature)
                      74

(1 rows)

[SiyaoIsHiding]

I think I can fix this. @kmuthukk May you please give any hints, e.g. file name, class name, function name, that will help me navigate to the cause of the issue?