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.
I like this change, but let's not print the exit code message if we didn't try to kill mongo.