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The MultiSocket is obsolete technology which is superceded by a more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
Set version to 0.26.4-rc1
Revert "Add [overlay] configuration section (experimental):"

This reverts commit 856fd9d69f9b5d114982d9de361bcdf5ecbba505.
Set version to 0.26.2-rc2
Add missing includes
Get rid of warnings in rippled and third-party code.

* Fix warnings in rippled code.
* Change Sconstruct to suppress specific warnings for individual
  third-party packages.
* Clean Sconstruct by removing repetition.