Support restore data from TiCDC log

[3pointer]

Feature Request

Describe your feature request related problem:

Part of #325 , After pingcap/ticdc#768 TiCDC support external storage. BR need to restore data from TiCDC output log.

Describe the feature you'd like:

Support restore data from TiCDC log.

Describe alternatives you've considered:

Since there are two kind of TiCDC logs KV changes and DDL events.
For KV changes, BR should apply log in multi-thread.

Workload estimation(person-day)

• Design: 2
• Develop: 4
• QA: 5
• Document: 3

[kennytm]

note about merging Lightning and BR:

I oppose blindly combining the two repositories, as each has accumulated a long and diverging history. But I welcome efforts to move the KV Encoder and Local Backend to BR, and let Lightning reuse them as a library (similar to the current Split & Scatter).