What problem does this PR solve?

Improves MySQL compatibility of SHOW commands, plus an enhancement to derive info from infoschema to reduce duplication.

What is changed and how it works?

Lots of small compatibility bugs:

• Map SHOW ENGINES and SHOW TABLE STATUS to infoschema (saves duplication)
• Fix return type of SHOW TABLE STATUS column
• Add privilege checks before checking if DB exists as in MySQL.
• Use correct MySQL error codes for errors, privilege checks
• Added partition info to infoschema.

Check List

Tests

• Unit test
• Manual test

Code changes

• Minimal

Side effects

• Breaking backward compatibility

The added privilege checks may break incorrect apps.

Related changes

• Need to update the tidb-ansible repository
• Need to be included in the release note

This change is