Optimize DataWatcher

The lock in DataWatcher is used to prevent concurrent modifications
to the 'd' field (entries in MCP). However any modifications to
this map only occur on initialization of an Entity in its
constructor. This modification is write-locked.

Every other access is through a readlock, which allows
the threads to pass if there is no thread holding the
writelock.

Since the writelock is only obtained in the constructor
of the Entity, the further readlocks are actually
useless (which get obtained on set, get, etc calls).

The entries field ('d' currently) has also been declared as
Int2ObjectOpenHashMap to avoid autoboxing on put(), get(), etc
calls.

Spigot-Server-Patches/Use-Optimized-Collections.patch

@@ -13,7 +13,7 @@ These collections are super fast as seen
 http://java-performance.info/hashmap-overview-jdk-fastutil-goldman-sachs-hppc-koloboke-trove-january-2015/
 
 diff --git a/src/main/java/net/minecraft/server/DataWatcher.java b/src/main/java/net/minecraft/server/DataWatcher.java
-index 79a240cd1..6c259effb 100644
+index 79a240cd1d..f224043d8e 100644
 --- a/src/main/java/net/minecraft/server/DataWatcher.java
 +++ b/src/main/java/net/minecraft/server/DataWatcher.java
 @@ -0,0 +0,0 @@ import java.util.Map;
@@ -29,7 +29,7 @@ index 79a240cd1..6c259effb 100644
      private static final Map<Class<? extends Entity>, Integer> b = Maps.newHashMap();
      private final Entity c;
 -    private final Map<Integer, DataWatcher.Item<?>> d = Maps.newHashMap();
-+    private final Map<Integer, DataWatcher.Item<?>> d = new Int2ObjectOpenHashMap<>(); // Paper
++    private final Int2ObjectOpenHashMap<DataWatcher.Item<?>> d = new Int2ObjectOpenHashMap<>(); // Paper
      private final ReadWriteLock e = new ReentrantReadWriteLock();
      private boolean f = true;
      private boolean g;