feat: add IF NOT EXISTS config for migrator

[zhtiyissr]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

When migrator needs to be executed concurrently, it is not enough to judge by HasTable(), which will cause CreateTable() to return the table is already exist error. To avoid this error, hopefully use the IF NOT EXISTS statement. This change is forward compatible and will not cause changes to existing code.

User Case Description

Assume that in an APP, user login, registration, browsing, click and other events need to be processed, and these events need to be recorded in the table, then the table needs to be dynamically divided according to user_id. In a distributed scenario, when browsing and clicking events are processed concurrently, When, it may cause Migrator to be abnormal.

[jinzhu]

you can use AutoMigrate in this case, and this pr seems doesn't works as explained, so going to close it for now.