Move the common logic together with BR.

Feature Request

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

According to https://github.com/pingcap/br/pull/482#discussion_r488934395, BR and CDC both have the same variables and interfaces in different repo which increase the maintenance cos.

Describe the feature you'd like:

we can move the decoder logic together by

1. import BR cause CDC already import BR as lib.
2. move decoder into a third-part repo.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: