[native] Differentiate number of drivers and number of splits in stats reporting

[Yuhta]

Presto Java reports number of drivers to number of splits in Presto UI because split and driver are 1 to 1 mapping relationship. This is not true in Prestissimo where 1 driver handles many splits. We should extend the API (protocol::TaskStats and Presto coordinator UI) to have separate splits and drivers information.

[aditi-pandit]

@ethanyzhang

[aaneja]

Duplicate of #23441