Refactor to use FileTime API

[jorsol]

The reproducible builds feature uses the last modification date with the java.util.Date type, apart from all the flaws that the Date API has, the most representative type for this should be java.nio.file.attribute.FileTime, this API is available since Java 7 as part of the NIO.2 and should be preferred for this kind of use.

Proposal:
All the java.util.Date should be deprecated (maybe removed in the next major release 5.0?) and introduce a new set of alternatives using FileTime.