Gatekeeper server code

This is the basic gatekeeper server code.  At the time of creation of this PR, it includes the mongo interactions and the http server code with tests for each.

It does not have the initial "main" that starts up the server nor start.sh or build.sh.  Those are next on the list.

@kentquirk @jh-bate I'd like to get you guys to look over the mongo and http interactions while I'm doing the final wiring.  If everyone agrees to this, I might be able to actually integrate it before the end of the week and avoid an annoying migration...