The existing CNode::addrLocal member is revealed to the user,
as an address string, similar to the existing "addr" field.
Instead of showing garbage or empty string,
it simply will not appear in the output if local address not known yet.
New RPC "ping" command to request ping.
Implemented "pong" message handler.
New "pingtime" field in getpeerinfo, to provide results to user.
New "pingwait" field, to show pings still in flight, to better see newly lagging peers.
Josh Lehan