coprocessor/scanner: set key_only=true if needed

[AndreMouche]

@BusyJay @hicqu @hanfei1991 PTAL

[hicqu]

LGTM.

[ngaut]

/run-all-tests

[zhangjinpeng87]

ci failed.

[BusyJay]

What about handle?

[AndreMouche]

handle wouldn't appear in self.col_ids since it was filtered on L47 @BusyJay

[AndreMouche]

/run-all-tests

[hanfei1991]

LGTM

[AndreMouche]

Performance test result（with long value in default cf）
tidb branch(pingcap/tidb#5447):

in master

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 31.44 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 31.54 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 31.30 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 31.19 sec)

current branch

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 1.98 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 2.04 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 2.04 sec)

MySQL [test]> select count(*) from table10;
+-----------+
| count(*)  |
+-----------+
| 264488600 |
+-----------+
1 row in set (1 min 1.92 sec)