An Easter basket bearing gifts! (#191)

* Delay switch to new server until after JoinGame is sent.

Unfortunately, in some cases (especially vanilla Minecraft) some login
disconnects are sent after ServerLoginSuccess but before JoinGame.
We've been using ServerLoginSuccess but it has caused too many problems.
Now Velocity will switch to a stripped-down version of the play session
handler until JoinGame is received. This handler does very little by
itself: it simply forwards plugin messages (for Forge) and waits for the
JoinGame packet from the server.

This is an initial version: only vanilla Minecraft 1.12.2 was tested.
However this is the way Waterfall without entity rewriting does server
switches (which, in turn, is inherited from BungeeCord).

* Move to Gradle 5 and Error Prone.

settings.gradle

@@ -6,6 +6,4 @@ include(
 )
 findProject(':api')?.name = 'velocity-api'
 findProject(':proxy')?.name = 'velocity-proxy'
-findProject(':native')?.name = 'velocity-native'
-
-enableFeaturePreview('STABLE_PUBLISHING')
+findProject(':native')?.name = 'velocity-native'