Migrate to Go Modules

[itchyny]

This is a pull request to migrate from glide to Go Modules. WIP blocked by jpillora/go-tcp-proxy. Since tcp-proxy can be installed as a command, it was excluded from go.sum.

• I switched the credits generation from script/credits to gocredits. The tool parses go.sum. Note that two libraries are added to ./fireworq -credits, https://github.com/golang/mock (Apache License Version 2.0) and https://github.com/phayes/freeport (BSD 3-Clause).
• Previously gomock was fix to 1.2.0 at Use a stable version of golang/mock #24, but the version is not compatible with Go Modules mockgen source mode does not work with go1.11 modules golang/mock#230. When I updated the version of gomock, I see another problem that mock generation fails due to compilation errors, which is related to mockgen: Source mode compiles package golang/mock#275. I changed the way to generate mocks to reflect mode, which does not require the package to pass compilation before mock generation. Anyway make deps does not update the version of gomock so please be sure to update gomock manually.

[tarao]

Thank you very much!