On-demand GroupVPN

Add another option to create P2P connection. There used to be one
option that is creating P2P when it login to XMPP.  Now, it can
create P2P when the packet is detected in the tapdevice that is
destined to remote node.

 - On-demand mode is configurable through config file
 - If the connection is inactive certain threshold, it trims the
   connection. This threshold is configurable through config file
Make repeated codes in functions

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Merge pull request #9 from kyuhojeong/on-demand

On-demand GroupVPN