Add support for NAT traversal techniques

[yciabaud]

Here is an implementation of NAT traversal as discussed in #195 using https://github.com/indutny/node-nat-upnp and https://github.com/TooTallNate/node-nat-pmp.

I have also ecountered https://github.com/level451/pmp that claims to make both UPnP ad NAT-PMP but I don't have tested it.

This feature is implemented in a new module for node env only and provided by a single instance shared by the webtorrent clients because NAT-PMP needs to open a specific port to work.

I checked the UPnP part with my router by it does not support NAT-PMP protocol.

The mappings are sent both with UPnP and NAT-PMP whe available and refreshed every 30 min.

Reviews are welcome!

[yciabaud]

Using my patch TooTallNate/node-nat-pmp#13 does the trick, we should wait for it to be merged but I have no feedback ATM.

[heroboy]

What protocol port you used to map? As I know dht uses UDP. self._natTraversal.portMapping seems mapping TCP port default.

[stale]

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