TAJO-1140: Separate TajoClient into fine grained parts.

Closes #213
TAJO-1114: Improve ConfVars (SessionVar) to take a validator interface to check its input. (Jihun Kang via hyunsik)

Closes #218
Merge branch 'master' of http://git-wip-us.apache.org/repos/asf/tajo into index_support