TAJO-1140: Separate TajoClient into fine grained parts.

This is a code clean up and refactoring patch. This patch main does as follows:

* Separate TajoClient into SessionConnection, QueryClient, and CatalogAdminClient
* Keep backward compatibility
* Separate QueryClient into its interface and its implementation
* Separate CatalogAdminClient into its interface and its implementation
* Rename TajoConnection to JdbcConnection