Run discovery DHT behind nat

[arnetheduck]

This ticket describes issues with the current code, when running behind a nat:

• node A runs on port 50000 on network X.0
• node B runs on port 50001 on network X.0
• node C runs on port 50002 on network Y.0
• to node C, node A runs on network X.1, behind a NAT

A and B connect to each other on local network - they will have each others 'X.0' addresses in the DHT. Now C connects to A because it had a bootstrap address with X.1:50000, and is unable to connect to node B, because there is no DHT entry with X.1 - A only sees B as X.0:50001

                                 FIREWALL
                                 +----+
                                 |    |
                                 |    |       +-----------------+
                                 |    |       |                 |
                                 |    |       |                 |
                                 |    |       |      A          |
                                 |    +-------+                 |
                                 |    |       |                 |
                                 |    |       |                 |
                                 |    |       |     X.0:50000   |
        +-----------+            |    |       +-----------------+
        |           |            |    |                |
        |           |            |    |                |
        |           |            |    |                |
        |   C       +------------+X.1 |                |
        |           |            |    |                |
        |           |            |    |                |
        |           |            |    |                |
        +-----------+            |    |                |
                                 |    |                |
                                 |    |       +-----------------+
                                 |    |       |     X.0:50001   |
                                 |    |       |                 |
                                 |    |       |       B         |
                                 |    +-------+                 |
                                 |    |       |                 |
                                 |    |       |                 |
                                 |    |       |                 |
                                 |    |       |                 |
                                 |    |       +-----------------+
                                 |    |
                                 +----+