Pinot connector: by default turn on push down top n queries

[xiangfu0]

This config was off as Pinot version < 0.4 doesn't support aggregation group by order by, so the order by can not be pushdown. We need to retrieve many groups then let presto order it.

Since Pinot supports ORDER BY now, this clause can now be pushed down by default.

For example, for presto query
SELECT Origin, sum(AirTime) FROM airlineStats GROUP BY Origin ORDER BY Origin LIMIT 10
the generated Pinot queries to push down are:

When the config is turned off (pinot.pushdown-topn-broker-queries=false):

GeneratedPinotQuery=SELECT Origin, sum(AirTime) FROM airlineStats GROUP BY Origin LIMIT 1000

After the config is turned on (pinot.pushdown-topn-broker-queries=true):

GeneratedPinotQuery=SELECT Origin, sum(AirTime) FROM airlineStats GROUP BY Origin ORDER BY Origin LIMIT 10

== RELEASE NOTES ==

Pinot Changes
* Turn on config `pinot.pushdown-topn-broker-queries` by default to push down aggregation group by order by queries to improve query performance.

[xiangfu0]

@highker