[Bleeding] Added a Metadata framework for Entities, Blocks, and Worlds

This metadata implementation has the following features:

- All metadata is lazy. Metadata values are not actually computed until another plugin requests them. Memory and CPU are conserved by not computing and storing unnecessary metadata values.

- All metadata is cached. Once a metadata value is computed its value is cached in the metadata store to prevent further unnecessary computation. An invalidation mechanism is provided to flush the cache and force recompilation of metadata values.

- All metadata is stored in basic data types. Convenience methods in the MetadataValue class allow for the conversion of metadata data types when possible. Restricting metadata to basic data types prevents the accidental linking of large object graphs into metadata. Metadata is persistent across the lifetime of the application and adding large object graphs would damage garbage collector performance.

- Metadata access is thread safe. Care has been taken to protect the internal data structures and access them in a thread safe manner.

- Metadata is exposed for all objects that descend from Entity, Block, and World. All Entity and World metadata is stored at the Server  level and all Block metadata is stored at the World level.

- Metadata is NOT keyed on references to original objects - instead metadata is keyed off of unique fields within those objects. Doing this allows metadata to exist for blocks that are in chunks not currently in memory. Additionally, Player objects are keyed off of player name so that Player metadata remains consistent between logins.

- Metadata convenience methods have been added to all Entities, Players, Blocks, BlockStates, and World allowing direct access to an individual instance's metadata.

- Players and OfflinePlayers share a single metadata store, allowing player metadata to be manipulated regardless of the player's current online status.

By: rmichela <deltahat@gmail.com>

paper-api/src/main/java/org/bukkit/metadata/Metadatable.java

@@ -0,0 +1,42 @@
+package org.bukkit.metadata;
+
+import org.bukkit.plugin.Plugin;
+
+import java.util.List;
+
+/**
+ * This interface is implemented by all objects that can provide metadata about themselves.
+ */
+public interface Metadatable {
+    /**
+     * Sets a metadata value in the implementing object's metadata store.
+     *
+     * @param metadataKey A unique key to identify this metadata.
+     * @param newMetadataValue The metadata value to apply.
+     */
+    public void setMetadata(String metadataKey, MetadataValue newMetadataValue);
+
+    /**
+     * Returns a list of previously set metadata values from the implementing object's metadata store.
+     *
+     * @param metadataKey the unique metadata key being sought.
+     * @return A list of values, one for each plugin that has set the requested value.
+     */
+    public List<MetadataValue> getMetadata(String metadataKey);
+
+    /**
+     * Tests to see whether the implementing object contains the given metadata value in its metadata store.
+     *
+     * @param metadataKey the unique metadata key being queried.
+     * @return the existence of the metadataKey within subject.
+     */
+    public boolean hasMetadata(String metadataKey);
+
+    /**
+     * Removes the given metadata value from the implementing object's metadata store.
+     *
+     * @param metadataKey the unique metadata key identifying the metadata to remove.
+     * @param owningPlugin This plugin's metadata value will be removed. All other values will be left untouched.
+     */
+    public void removeMetadata(String metadataKey, Plugin owningPlugin);
+}