Alignment issues on at least ARM

```
../src/torrent/net/socket_address_key.h:41:3: warning: field  within 'torrent::socket_address_key' is less aligned than 'torrent::socket_address_key::(anonymous union at ../src/torrent/net/socket_address_key.h:41:3)' and is usually due to 'torrent::socket_address_key' being packed, which can lead to unaligned accesses [-Wunaligned-access
```

That union should probably be using std::variant.