ignore p2p messages of unknown types

[cc32d9]

In current implementation, if a node receives a message of unknown type, it throws an error and disconnects the socket:

mandel/plugins/net_plugin/net_plugin.cpp

Line 2588 in fdc7e00

fc::raw::unpack( ds, msg );

If the plugin is modified to ignore unknown message types, it would allow extending the protocol without having to upgrade the whole network and activate a feature. One of examples is the fast finality proposal which would send block confirmations in a new type of message.