remove mydumper dependence and use a Go implementation directly

[siddontang]

Feature Request

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

Now we must install mydumper explicitly to let DM to support full backup, it is not easy to operate.

why not let DM support dump directly?

Describe the feature you'd like:

Refer to https://github.com/xelabs/go-mydumper

Seem we can use the above Go lib directly, maybe the only risk is the stability of the lib. But the dump logic is easy, and I think we can hold the lib easily.

Describe alternatives you've considered:
Maybe reinvent the wheel, it is not easy work.

Teachability, Documentation, Adoption, Migration Strategy:
No

[kennytm]

Note that go-mydumper is GPL'ed so we can't use that project as a library. We could still bundle it as a separate executable and run it as a sub process but there won't be much advantage compared with the current mydumper in terms of integration.

[csuzhangxc]

already replaced with dumpling in #540.