Fix test job timeouts in Travis CI

Recent changes in the way we're closing threads in Java code during REEF driver shutdown seem to have introduced a bug in this area. We observe transient test job timeouts in Travis CI: typically one test job takes 39-41 minutes, the limit on job duration is 50 minutes, and we're seeing test jobs hitting the limit and timing out. There is no test failure reported in such cases, so I suspect there is some runaway unaccounted for thread or an entire test which fails to complete properly.