feat concurrent chunk data

[shaohk]

Description

This PR Supports concurrent copying of chunk blocks during each time slice of copying the existing data.

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[shaohk]

MySQL insert ... select ... does not permit concurrent.

https://dev.mysql.com/doc/refman/5.7/en/insert-select.html

[shaohk]

When the unique index used for copying blocks includes an auto-increment column, concurrency is blocked on the auto-inc lock. If the unique index used does not include an auto-increment column, concurrency can proceed.