Refactor Overlay and add [overlay] config section (experimental):
These changes prepare Overlay for the Universal Port and Hub and Spoke
features.

* Add [overlay configuration section:
The [overlay] 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.

Other changes:
* Use _MSC_VER to detect Visual Studio
* Use ssl_bundle in Overlay::Peer
* Use shared_ptr to SSL context in Overlay:
* Removed undocumented PEER_SSL_CIPHER_LIST configuration setting
* Add Section::name: The Section object now stores its name for better diagnostic messages.
Refactor HTTP::Server to support Universal Port:
These changes are necessary to support the Universal port feature. Synopsis:

* Persist HTTP peer io_service::work lifetime:
This simplification eliminates any potential for bugs caused by incorrect
lifetime management of the io_service::work object.

* Restructure Door to prevent data races, and handle clean exit:
The Server, Door, Door::detector, and Peer objects work together to
correctly implement graceful stop and destructors that block until
all child objects have been destroyed.

Cleanups:
* De-pimpl HTTP::Server
* Rename ServerImpl data members
* Tidy up HTTP::Port interface