TAJO-588: In some case, leaf task of DefaultTaskScheduler are not distributed execution. (jinho)
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/incubator-tajo into TAJO-583
fixed testsortexec.
Merge branch 'TAJO-583' of github.com:hyunsik/incubator-tajo into TAJO-583
rewrite reaplceNode.