Implement lanscatter_proxy

[elonen]

Proxy command (see "site-to-site distribution" on README) should be simple to implement:

• Create proxynode.py and add it as a endpoint in setup.py.
• Add support for CLI options in common.py
• Import peernode and masternode in proxynode.py. Then launch both in separate asyncio tasks.
• Customize log writer so it prepends "{PEER}" or "{MASTER}" depending on which process the log line came from.
• Add integration tests
• Add "proxy mode" in GUI settings page. It should enable both peer and master settings panes.