Performance improvement (MessageHandler)

Improved the performance of RISE V2G by converting the MessageHandler into a Singleton. When instantiating the MessageHandler, the JAXB Context is set which is a pretty time-consuming task. Before the improvement, the MessageHandler was instantiated by the V2GCommunicationSessionHandlerSECC and the V2GCommunicationSessionHandlerSECC class.
Additionally, the MessageHandler was instantiated each time a new communication session was initiated by the EVCC.

Now, MessageHandler is initialized only once at startup of RISE V2G, saving a few seconds processing time on a slow embedded controller.
Credit goes to Advantics for pointing this out! Thanks. :)

Further changes: minor editorial edits and an additional logging message in the ConnectionHandler stating the length of the V2GTP payload as stated by the V2GTP header (helps for debugging purposes).