*: support stream agg on TiKV

[zimulala]

Relate to tikv/tikv#2714

[shenli]

LGTM

[zimulala]

Using 3 TiKV, 1 PD, 1 TiDB, the 3 TiKV are on different servers.
The table schema as follows:

Create Table: CREATE TABLE `lineitem` (
  `L_ORDERKEY` int(11) NOT NULL,
  `L_PARTKEY` int(11) NOT NULL,
  `L_SUPPKEY` int(11) NOT NULL,
  `L_LINENUMBER` int(11) NOT NULL,
  `L_QUANTITY` decimal(15,2) NOT NULL,
  `L_EXTENDEDPRICE` decimal(15,2) NOT NULL,
  `L_DISCOUNT` decimal(15,2) NOT NULL,
  `L_TAX` decimal(15,2) NOT NULL,
  `L_RETURNFLAG` char(1) NOT NULL,
  `L_LINESTATUS` char(1) NOT NULL,
  `L_SHIPDATE` date NOT NULL,
  `L_COMMITDATE` date NOT NULL,
  `L_RECEIPTDATE` date NOT NULL,
  `L_SHIPINSTRUCT` char(25) NOT NULL,
  `L_SHIPMODE` char(10) NOT NULL,
  `L_COMMENT` varchar(44) NOT NULL,
  PRIMARY KEY (`L_ORDERKEY`,`L_LINENUMBER`),
  CONSTRAINT `LINEITEM_FK1` FOREIGN KEY (`L_ORDERKEY`) REFERENCES `orders` (`O_ORDERKEY`),
  CONSTRAINT `LINEITEM_FK2` FOREIGN KEY (`L_PARTKEY`,`L_SUPPKEY`) REFERENCES `partsupp` (`PS_PARTKEY`,`PS_SUPPKEY`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin

	rows:6001215 && groups:1.5m && current PR	rows:6001215 && groups:1.5m && before	percentage decrease	rows:59986052 && groups:15m && current PR	rows:59986052 && groups:15m && before	percentage decrease
select count(L_ORDERKEY), sum(L_LINENUMBER) from lineitem group by L_ORDERKEY;	6.43 sec	7.27 sec	11.6%	31.05s	69.76s	55.5%
	6.53 sec	7.36 sec	11.3%
---	---	---	---	---	---	---
select count(L_ORDERKEY) from lineitem group by L_ORDERKEY;	4.12 sec	4.84 sec	14.9%	19.78s	49.03s	59.7%
	4.22 sec	4.61 sec	8.5%
---	---	---	---	---	---	---
select sum(L_ORDERKEY) from lineitem group by L_ORDERKEY;	5.00 sec	5.51 sec	9.3%	26.76s	52.55s
	5.15 sec	5.72 sec
---	---	---	---	---	---	---
select avg(L_LINENUMBER) from lineitem group by L_ORDERKEY;	6.04 sec	6.21 sec	2.8%	32.26s	61.08s	49.1%
	6.15 sec	6.39 sec	3.8%
---	---	---	---	---	---	---
select avg(L_ORDERKEY) from lineitem group by L_ORDERKEY;	5.97 sec	6.03 sec	1.0%	35.20s	60.59s	41.9%
	6.20 sec	6.30 sec	1.6%
---	---	---	---	---	---	---

[shenli]

Can we merge this now?

[winoros]

/run-integration-common-test

[winoros]

/run-integration-common-test tidb-test=pr/453 tidb-private-test=pr/453

[winoros]

/run-sqllogic-test

[zimulala]

/run-integration-common-test tidb-test=pr/453 tidb-private-test=pr/453 tikv=pr/2762

[zimulala]

PTAL @winoros

[winoros]

LGTM