rUDP keep connection with new node using existing node-id

[Fraser999]

Migrated from maidsafe-archive/MaidSafe#99

Scenario:
Two bootstrap node exist, third node successfully join the n/w and all three nodes successfully populated their routing tables. Now fourth node join n/w using existing third node key. The bootstrap node which is used as a peer by fourth node break connection with existing third node and make connection with fourth node and keep the connection for fourth node. Third node still have its connection with second bootstrap node. So both the node with duplicate node-id exist in network, in actual bootstrap should refuse the new fourth node connection and keep maintain the existing third node connection.