Do not allow set tidb_auto_analyze_ratio to 0

[Rustin170506]

Enhancement

Currently, if you set tidb_auto_analyze_ratio to 0, we will disable auto-analysis based on row changes, but we will still process the newly added indexes and tables.

This behavior is difficult to understand. I suppose most users will think 0 means we always need to analyze the table, no matter how many rows have been updated.

The historic reason we introduced it is that before #34643 this PR, there was no way to disable the auto-analyze feature without restarting the server. So we chose this special value to allow users to disable the auto-analysis feature. But now enable_tidb_auto_analyze is a global session variable. So we don't depend on this value.

So Roger and I suggest getting rid of this old behavior. No longer allow users to set it to zero.

However, I am not sure how to deal with the existing users' settings.

I think we have the following choices:

1. Help users to set enable_tidb_auto_analyze to off if the user's tidb_anto_analyze_ratio is 0.
2. Keep the old implementation and also keep the user's settings during the upgrade process. But we don't allow changing this value to 0 anymore.

[songrijie]

options 2 looks fine to me.

I believe the case setting tidb_anto_analyze_ratio to 0 is rare.

In addition, we can add extra comment in user doc of tidb_anto_analyze_ratio. If users want to completely disable auto analyze, go for enable_tidb_auto_analyze instead of setting tidb_anto_analyze_ratio to 0.

[Rustin170506]

I am working on this.