Rework addnode behaviour

• Use CNode::addeName to track whether a connection to a name is already open
  • A new connection to a previously-connected by-name addednode is only opened when the previous one closes (even if the name starts resolving to something else)
  • At most one connection is opened per addednode (even if the name resolves to multiple)
• Unify the code between ThreadOpenAddedNodeConnections and getaddednodeinfo
  • Information about open connections is always returned, and the dns argument becomes a dummy
  • An IP address and inbound/outbound is only reported for the (at most 1) open connection