Improve maven startup time

[gnodet]

This saves 15-20% on the startup time on the maven project for example.
On bigger projects, most of time is spent in loading projects, so this becomes negligible.

[rmannibucau]

OK, ideally if we need more init we can use a global shared forkjoin pool (instead of ExecutorService executor = Executors.newSingleThreadExecutor(); in jline module) but good enough for now