Support Golang 1.12 and use `module` to manage the 3rd party dependences instead of `dep`

[bill828]

Currently, Golang 1.10 is recommended, and Golang 1.12 has a built-in package management tool module.

We may need to use this instead of the 3rd party management tool dep to follow the official behavior.

To use this tool, we need to use Golang 1.12 for building and testing.

[ahrtr]

I suggest to hold off this change until golang 1.13, because go module will be the default dependency management tool at that time. Let's wait and see.

[matthewbaldwin]

Looks like this is an issue that still needs to be addressed. Can the 1.12 branch be merged?