[MCL-599] IllegalStateException on login during assets download

Steps to reproduce:
1. Clear your .minecraft folder
2. Head to the launcher, and while the resources are downloading:
a. Log-in using wrong account information (I used my MC username)
b. Log-in using right account information

This would be the result (after all resources finishes downloading):

Trying to log in...
Couldn't log in
java.lang.IllegalStateException: Cannot change username whilst logged in & online
	at net.minecraft.launcher.authentication.BaseAuthenticationService.setUsername(BaseAuthenticationService.java:85)
	at net.minecraft.launcher.ui.sidebar.login.NotLoggedInForm$1.run(NotLoggedInForm.java:157)
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
	at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
	at java.util.concurrent.FutureTask.run(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

Also, I'm surprised that the launcher is not async. Why is that?