Revise plan stats on changing partition prune mode

[Yisaer]

Enhancement

Currently, if we change tidb_partition_mode into dynamic from static, and execute query for a partition table without analyzing table for generating global stats. Optimizer will think the table stats as pseudo and provide a bad plan.

To revise this behavior, Optimizer can still think table stats as static for the partition table which has no global stats.

[mjonss]

@Yisaer, how about we always create both per partition and global statistics for partitioned tables, as long as we support both static and dynamic tidb_partition_prune_mode?

[Yisaer]

@mjonss I think this is a good idea, partition table can be always generated global stats and partition stats without considering prune mode. prune_mode should only affect the behavior of the optimizer, instead of analyzing. I will discuss this idea with PM later.