Strings: string related utility methods moved from the Files class here.
Files: removed, as it actually only served for string utilities. Duration: added await() and awaitUninterruptibly() methods. Duration: softSleep() -> sleepUninterruptibly() for consistency. Duration: timedWait(), timedJoin() -> wait(), join() since the time is implicit. Duration: moved to the ch.usi.dag.util package in src-util. Job: remove duplicated __streamToString() and use Strings.drainStream() instead. Job: use the generic awaitUninterruptibly() in waitFor(). src-test: reflect the above method and class movement.