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/Async-Chunk-Loading-and-Generation.patch

@@ -392,7 +392,7 @@ index 0000000000..5c77b6e8e1
 +
 +}
 diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
-index fe40d62692..33d52447be 100644
+index edfcb107bd..cb99888707 100644
 --- a/src/main/java/net/minecraft/server/Chunk.java
 +++ b/src/main/java/net/minecraft/server/Chunk.java
 @@ -0,0 +0,0 @@ public class Chunk implements IChunkAccess {
@@ -1736,7 +1736,7 @@ index f87182b5c4..574930f5fe 100644
      }
  
 diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index e52e4bb458..13f69f1b82 100644
+index a2559f0c19..bbcedb8fc7 100644
 --- a/src/main/java/net/minecraft/server/World.java
 +++ b/src/main/java/net/minecraft/server/World.java
 @@ -0,0 +0,0 @@ import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
@@ -1980,7 +1980,7 @@ index fa99fe0146..4f49786aa3 100644
                  }
              }
 diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
-index 14905fceb0..c0aeccffbf 100644
+index ad3fea9c97..0a76482638 100644
 --- a/src/main/java/net/minecraft/server/WorldServer.java
 +++ b/src/main/java/net/minecraft/server/WorldServer.java
 @@ -0,0 +0,0 @@ public class WorldServer extends World implements IAsyncTaskHandler {