Switch to Java 8

[ffissore]

With #3312 merged, every IDE for every OS bundles the JVM. We can now switch to java 8 without forcing installation of an updated JVM.

[jverne]

Part of the move to Java 8 might consider a bulk edit to get rid of the warnings about bare "_" identifiers.

[ffissore]

Indeed

[ffissore]

Fixed by 91df9b1. _ identifiers tracked at #3344