Improve primary to route result cache

[yanhuqing666]

Backgroud

Now only if query's filter contains primary key, we will add it to route cache.

Improve

For simple query，if result contains primary key， we can add it to route cache.

Effect

Not all cases need primary- route cache. Sometimes add to cache may cause performance reduction.
If no need, please do not setting primary key in table config.

[yanhuqing666]

It suitable for this case：
sharding column is column1 and primarykey is column2.

[yanhuqing666]

https://github.com/actiontech/dble-docs-cn/blob/develop/2.Function/2.16_cache.md

[yexiaoli88]

#969

[yexiaoli88]

verified on dble version: 5.6.29-dble-9.9.9.9-4e4db72053f13237a444c8fcc986db8660e38880-20190129085254

auto_test position: schema_attribute.feature scenario #6 #7