Parallelize Gradle build

[lupino3]

https://docs.gradle.org/6.1.1/userguide/multi_project_builds.html#sec:parallel_execution

For example, we could have the following projects:

• shared (core code, tests)
• docs (compilation of html/pdf code)
• war (GWT code)
• jar

This is just a strawman proposal, there might be a better way to parallelize the build. This would mostly benefit local builds, the CI pipeline is fast enough and there are better ways to improve parallelism there (by properly using jobs, stages, etc.).

[lupino3]

https://docs.gradle.org/current/userguide/declaring_dependencies_between_subprojects.html#declaring_dependencies_between_subprojects