TAJO-1038: Remove use of Builder variable in Schema.

Closes #140
TAJO-1034: Reduce Explicit Use of JVM Internal Class. (Jihun Kang via hyunsik)
TAJO-1024: RpcConnectionPool::getConnection can cause NPE at initialization.

Closes #139
TAJO-1037: KillQuery hang in subquery init state. (jinho)

Closes #137
TAJO-1015: Add executionblock event in worker. (jinho)

Closes #124
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-907

Conflicts:
	tajo-core/src/main/java/org/apache/tajo/worker/TajoWorker.java
	tajo-core/src/main/java/org/apache/tajo/worker/Task.java