TAJO-1195: Remove unused CachedDNSResolver Class. (DaeMyung Kang via jaehwa)

Closes #253
TAJO-1197: Unit test failed: unable to create new native thread. (jinho)

Closes #255
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-1181