Optimize webpack build time

[slaweet]

Expected behaviour

I believe there are ways to speed up webpack build.
For example, we build all the dependencies each time, while most of the time they don't change. There is Webpack DLL plugin to prevent this.

Actual behaviour

npm run build takes about 1 minute
npm run test takes about 2 minutes

Steps to reproduce

run npm run build or npm run test

[slaweet]

I already tried to improve build time with hard-source-webpack-plugin in #956 and it helps a bit as you can see with consecutive builds here https://jenkins.lisk.io/job/lisk-nano/job/development/

It saves the cache to node_modules folder which we cache across builds (look for
HardSourceWebpackPlugin is writing to a new confighash path for the first time in https://jenkins.lisk.io/job/lisk-nano/job/development/87/consoleFull )
But for some reason, it uses a different hash on another branch but with exact same code: https://jenkins.lisk.io/job/lisk-nano/job/1.3.0/3/consoleFull