Kill child on process.exit()
Kill child on process.exit()

Otherwise the java process stays around after running a node program
like this:

  const DynamoDbLocal = require('dynamodb-local');
  DynamoDbLocal.launch(8001, null, ['-sharedDb', '-inMemory'])
    .then(function() {
      process.exit(0);
    })

which happens in the wild. For example, some test frameworks behaves
like this when in so called watch mode and you type 'q' as in 'quit'.

doapp-ryanp/dynamodb-local