[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.py

@@ -19,7 +19,7 @@ install_requires = [
     "pyyaml==3.13",
     "raven==6.10.0",
     "raven-aiohttp==0.7.0",
-    "red-lavalink==0.2.2",
+    "red-lavalink==0.2.3",
     "schema==0.6.8",
     "websockets==7.0",
     "yarl==1.3.0",