Better enforce the Request lifetime.

Previously, the current request would just get a new EditSession when
one was created. Now, a Request is reset before and after:
 - a command is used and
 - an interact is fired with the platform
This means each action taken will get a single, non-reusable Request.

Note that this only applies to actions taken through the platform.
API users will not be using requests anyway, since things like Masks,
etc. will be constructed directly instead of being passed through the
platform's parsers and so on. (e.g. if a plugin loads a schematic into
the world with a mask, they should create the EditSession and mask it
directly, and not use that Mask again for another EditSession in another
World).

Also, get rid of a bunch of (some now-)unnecessary EditSession creation
during command dispatching.

Note that this also fixed the dynamic selection mask, which apparently
has been broken for some unknown amount of time.

worldedit-core/src/main/java/com/sk89q/worldedit/session/SessionManager.java

@@ -28,6 +28,7 @@ import com.sk89q.worldedit.LocalSession;
 import com.sk89q.worldedit.WorldEdit;
 import com.sk89q.worldedit.entity.Player;
 import com.sk89q.worldedit.event.platform.ConfigurationLoadEvent;
+import com.sk89q.worldedit.session.request.Request;
 import com.sk89q.worldedit.session.storage.JsonFileSessionStore;
 import com.sk89q.worldedit.session.storage.SessionStore;
 import com.sk89q.worldedit.session.storage.VoidStore;
@@ -151,6 +152,7 @@ public class SessionManager {
                 log.warn("Failed to load saved session", e);
                 session = new LocalSession();
             }
+            Request.request().setSession(session);
 
             session.setConfiguration(config);
             session.setBlockChangeLimit(config.defaultChangeLimit);
@@ -313,7 +315,7 @@ public class SessionManager {
     /**
      * Stores the owner of a session, the session, and the last active time.
      */
-    private static class SessionHolder {
+    private static final class SessionHolder {
         private final SessionKey key;
         private final LocalSession session;
         private long lastActive = System.currentTimeMillis();

worldedit-core/src/main/java/com/sk89q/worldedit/session/request/Request.java

@@ -35,6 +35,7 @@ public final class Request {
     private @Nullable World world;
     private @Nullable LocalSession session;
     private @Nullable EditSession editSession;
+    private boolean valid;
 
     private Request() {
     }
@@ -106,6 +107,20 @@ public final class Request {
      * Reset the current request and clear all fields.
      */
     public static void reset() {
+        request().invalidate();
         threadLocal.remove();
     }
+
+    /**
+     * Check if the current request object is still valid. Invalid requests may contain outdated values.
+     *
+     * @return true if the request is valid
+     */
+    public boolean isValid() {
+        return valid;
+    }
+
+    private void invalidate() {
+        valid = false;
+    }
 }

worldedit-core/src/main/java/com/sk89q/worldedit/session/request/RequestExtent.java

@@ -39,13 +39,13 @@ import java.util.List;
 
 public class RequestExtent implements Extent {
 
-    private EditSession extent;
+    private Request request;
 
     protected Extent getExtent() {
-        if (extent == null) {
-            extent = Request.request().getEditSession();
+        if (request == null || !request.isValid()) {
+            request = Request.request();
         }
-        return extent;
+        return request.getEditSession();
     }
 
     @Override
@@ -103,7 +103,7 @@ public class RequestExtent implements Extent {
     @Nullable
     public Operation commit() {
         Operation commit = getExtent().commit();
-        extent = null;
+        request = null;
         return commit;
     }
 }