Start Mongo in parallel with the build.

For freshly created apps, this commit reduces the time required to start the development server from 4.6s to 2.5s on my machine.

Not calling `findMongoAndKillItDead` unless Mongo fails to start shaves off a few hundred of those milliseconds.

cc @glasser 

Fixes #3010.
This wait has to occur between "build the app" and "run the app".  Either inject it into appRunner.start or (if it's easy to do) split appRunner.start into two methods.  Make sure the wait only happens on the initial build, not rebuilds, though!