Improvements to Logging Warnings/Dupe Entities - Resolves #1544

1) Removed "Regen" mode of Dupe UUID resolver, forced safe.
Some servers who updated before we had safe mode added still had this value.

There's really no reason to keep this mode, as we've seen that vanilla
triggers this often and 99.9999999% of cases will be an actual duplicate
that needs to be deleted.

2) Made Vanilla Debug messages about dupe UUIDs and dupe uuid resolve messages
only show up if the debug.entities flag is on. This will stop server owners
from panicing from seeing these logs, and stop opening bug reports on this,
only for us to tell you "don't worry about it".

3) Avoid adding entities to world that are already added to world.

This can be triggered by anything that causes an entity to be added
to the world during the chunk load process, such as chunk conversions.

Issue #1544 was a case of this.

4) Removed debug warning about ExpiringMap.

Nothing more I know to do about this anyways. We recover from it,
stop warning to reduce noise of issues to us.

Spigot-Server-Patches/Always-process-chunk-removal-in-removeEntity.patch

@@ -8,7 +8,7 @@ which can keep them in the chunk when they shouldnt be if done
 during entity ticking.
 
 diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index 81777fce64..597169b4cc 100644
+index b3eaac23a6..0331512468 100644
 --- a/src/main/java/net/minecraft/server/World.java
 +++ b/src/main/java/net/minecraft/server/World.java
 @@ -0,0 +0,0 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc