Implement solo mode without consensus enabled

This is an attempt to provide a simpler way to run doozerd in single node mode without hitting any of the consensus code paths.

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@soundcloud/iss 
Looking at this file as an outsider the name 'cl' is completely opaque.  The context offers no clues.  The same for 'l' and must listen.  'l' is also atrocious as a one letter variable name.  In many fonts, including the one I'm writing these comments in, it is (or is almost) completely indistinguishable from the letter I.  (Also, what does mustListen() produce?)