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.
we should probably set allowKilling to true in the self.multiple case (which is only used by a self-test, but it doesn't do any restarting)