[build system] speeding up maven build building only changed modules compared to master branch

[vaclavkosar]

I think it would be good idea to use gitflow-incremental-builder maven 
plugin for Spark builds. It saves resources by building only modules 
that are impacted by changes compared to git master branch via 
gitflow-incremental-builder maven plugin. For example if there is only a 
change introduced into on of files of spark-avro_2.11 then only that 
maven module and its maven dependencies and dependents would be build or 
tested. If there are no disagreements, I can submit a pull request for that.

Project URL: https://github.com/vackosar/gitflow-incremental-builder

Disclaimer: I originally created the project. But most of recent 
improvements and maintenance were deved by Falko.