Use process.exitCode instead of explicitly using process.exit

[shazron]

In aio app run, we have a SIGINT handler, and we explicitly call process.exit after trying to clean up our resource usage. We should just set the appropriate exit code and let the program naturally exit.

However, something is not being cleaned up properly, thus the program does not exit. Will need to investigate the issue further.

See discussion here: #343 (comment)