Reduce the time dumpling holds the FTWRL lock

[lichunzhu]

Feature Request

Is your feature request related to a problem? Please describe:

When dumpling MySQL, dumpling will use FTWRL to lock all the tables and release the lock after all tables have been dumped which will strongly affect the MySQL server.

Describe the feature you'd like:

In mydumper, they use a special way to confirm consistency and "lock time". The solution is: FTWRL -> start THREADS*transactions(WITH CONSISTENT SNAPSHOT) -> UNLOCK TABLES -> dump data
I think we can also do this in dumpling.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Optimization: