Mysql connector support parallel snapshot when there are no primary key for tables

[EMsnap]

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

Mysql connector now only support parallel snapshot for tables with primary key which means that
we can only use cdc-version1 to support multiple tables read when there are some tables are without primary key
Read multiple tables using just one subtask is pretty slow and unbearable for the users

Describe the solution you'd like

1、remove the constraint in mysql-cdc 2.0 for primary key
2、for those tables which don't contain primary key, split the whole table as a chunk during split stage
3、since there's no need for the SnapshotSplitReader to upsert binlog with the snapshot data for a whole table
split , just let the highwatermark equals to the lowwatermark to escape the upsert stage.

[zstraw]

@EMsnap Hi, EMsnap, what's the status of the issue? I find the PR only modify the codes in test.

[zstraw]

Hi, @leonardBang , could you spare some time to review the PR?

[ruanhang1993]

Closed by #2150.