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
I wonder why all cached variables are cleared before update.
Whenever a set command is executed, a new hash map instance is passed from the client.
(Please see SetCommand.updateSessionVariable().)