[Audio] Connect to lavalink in the background (#2460)

Also:
- restart and reconnect if connection settings change
  - shutdown and restart if not configured to use external
- show a message in [p]play et al. when the connection hasn't been made
- move the JAR download to manager so audio.py can access it
- only start if no process exists
- bump red-lavalink to 0.2.3

Resolves #2306

setup.cfg

@@ -40,7 +40,7 @@ install_requires =
     multidict==4.5.2
     python-levenshtein-wheels==0.13.1
     pyyaml==3.13
-    red-lavalink==0.2.2
+    red-lavalink==0.2.3
     schema==0.6.8
     websockets==7.0
     yarl==1.3.0