Limit expression thread count to processor count.

worldedit-core/src/main/java/com/sk89q/worldedit/internal/expression/Expression.java

@@ -74,7 +74,8 @@ import java.util.concurrent.TimeoutException;
 public class Expression {
 
     private static final ThreadLocal<Stack<Expression>> instance = new ThreadLocal<>();
-    private static final ExecutorService evalThread = Executors.newCachedThreadPool(
+    private static final ExecutorService evalThread = Executors.newFixedThreadPool(
+        Runtime.getRuntime().availableProcessors(),
         new ThreadFactoryBuilder()
             .setDaemon(true)
             .setNameFormat("worldedit-expression-eval-%d")