Fix the sequential build

Summary:
- When we start falcon so that it triggers a build instead of starting a server,
  we need to wait until the build completes before exiting.
- I also took care of using StdoutStreamConsumer instead of StreamServer because
  in this case, it has not been started properly (openPort is only called when
  we start the server).

Test Plan: ./falcon --graph makefile.json --log-level error --build

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D30