dm-worker: split DDL sync from sync function

[3pointer]

What problem does this PR solve?

Optimize sync function

What is changed and how it works?

Split sync DDL from sync() because sync DDL and sync DML don't have to many common logic.
Split them to make the code clear.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the dm/dm-ansible
• Need to be included in the release note

[codecov]

Codecov Report

Merging #289 into master will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master       #289   +/-   ##
===========================================
  Coverage   60.0717%   60.0717%           
===========================================
  Files           134        134           
  Lines         14769      14769           
===========================================
  Hits           8872       8872           
  Misses         5054       5054           
  Partials        843        843

[csuzhangxc]

@amyangfei @csuzhangxc PTAL

[amyangfei]

LGTM

[csuzhangxc]

LGTM