[SPARK-3632] ConnectionManager can run out of receive threads with authentication on

If you turn authentication on and you are using a lot of executors. There is a chance that all the of the threads in the handleMessageExecutor could be waiting to send a message because they are blocked waiting on authentication to happen. This can cause a temporary deadlock until the connection times out.

To fix it, I got rid of the wait/notify and use a single outbox but only send security messages from it until authentication has completed.

Author: Thomas Graves <tgraves@apache.org>

Closes #2484 from tgravescs/cm_threads_auth and squashes the following commits:

a0a961d [Thomas Graves] give it a type
b6bc80b [Thomas Graves] Rework comments
d6d4175 [Thomas Graves] update from comments
081b765 [Thomas Graves] cleanup
4d7f8f5 [Thomas Graves] Change to not use wait/notify while waiting for authentication