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