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.
Reviewers: @ximinez @mtrippled 
Why not remove the precompiler directive, and `#else` block entirely?