TAJO-1241: Change default client and table time zone behavior.

By default, TajoClient uses GMT as client time zone unless session variable TIMEZONE is specified. Also, by default Table uses GMT as table time zone unless table property timezone is specified.

This patch changes these default behavior as follows:
* TajoClient will use ``TimeZone.getDefault()`` by default.
* Table implicitly uses ``tajo.timezone`` of TajoConf by default.
    * In other words, this default time zone does not affect the table property in catalog.

I also added the documentation about time zone.
* http://people.apache.org/~hyunsik/timezone/time_zone.html
This is because we use synchronized feature against sessionVarsCache. In this case, if we replace the object, the synchronization will not work correctly. It will cause some concurrency problem.