Minimum example of a Ghostferry based application

[shuhaowu]

It would be nice to have, in the repository, a commented example of a minimum Ghostferry based application, as the existing copydb/sharding application is quite complex. This will allow beginners to quickly gain an understanding of how Ghostferry based applications should look like.

Having such an application will also benefit existing developers in testing POCs, as changing copydb/sharding could be a complicated manner.

I made a version of this when I'm testing some new features locally without depending on copydb and what not: https://gist.github.com/shuhaowu/5c48465040bda9d4143363f06c600c59. Anyone can try to convert this to a better example.

One thing we might also want to consider is to refactor copydb a little bit to make it more like a library so it can be customized more easily, as it exposes several "nice" features (like config file/filter building) that'll likely have to be reimplemented if a standalone app is to be created.

[shuhaowu]

The integration ferry could serve as such an example: https://github.com/Shopify/ghostferry/blob/master/test/lib/go/integrationferry.go