Don't use MultiSocket in Overlay:
The MultiSocket is obsolete technology which is superceded by a more
straightforward, template based implementation that is compatible with
boost::asio::coroutines. This removes support for the unused PROXY handshake
feature. After this change a large number of classes and source files may be
removed.
Add [overlay] configuration section (experimental):
This configuration section uses the new BasicConfig interface that supports
key-value pairs in the section. Some exposition is added to the example cfg
file. The new settings for overlay are related to the Hub and Spoke feature
which is currently in development. Production servers should not set
these configuration options, they are clearly marked experimental in the
example cfg file.

Conflicts:
	src/ripple/overlay/impl/OverlayImpl.cpp
	src/ripple/overlay/impl/OverlayImpl.h
	src/ripple/overlay/impl/PeerImp.cpp
	src/ripple/overlay/impl/PeerImp.h