Client NAT traversal 0.1

UPnP is now used to request a port on the NAT be forwarded to the local machine.
This obviously only works for NATs that support UPnP, and thus is not a panacea
for all NAT-related connectivity issues.

Notable hacks in this patch include a transmit/receive UDP socket pair to work
around current protocol limitations whereby the full node assumes its peer can
receive on the same UDP port it transmitted from.

multinode-demo/client.sh

@@ -20,5 +20,5 @@ rsync -vPz "$rsync_leader_url"/config/mint.json $SOLANA_CONFIG_DIR/
 
 # shellcheck disable=SC2086 # $solana_client_demo should not be quoted
 exec $solana_client_demo \
-  -n "$count" -l $SOLANA_CONFIG_DIR/leader.json -d \
+  -n "$count" -l $SOLANA_CONFIG_DIR/leader.json \
   < $SOLANA_CONFIG_DIR/mint.json