Add a hard cap of 200 threads to VelocityScheduler

This is a more realistic (generalized) solution for #943. Fundamentally, a plugin should not be spawning an unbounded number of asynchronous task execution units on demand from the user - an invariant Velocity itself enforces. However, since this practice is so commonplace, it makes sense that we would need to have some upper cap to at least make the practice safer than it currently is.

proxy/src/main/java/com/velocitypowered/proxy/scheduler/VelocityScheduler.java

@@ -34,8 +34,10 @@ import java.util.IdentityHashMap;
 import java.util.Set;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Consumer;
 import org.apache.logging.log4j.LogManager;
@@ -51,6 +53,8 @@ import org.checkerframework.checker.nullness.qual.Nullable;
  */
 public class VelocityScheduler implements Scheduler {
 
+  private static final int MAX_SCHEDULER_POOLED_THREAD_CAP = 200;
+
   private final PluginManager pluginManager;
   private final ExecutorService taskService;
   private final ScheduledExecutorService timerExecutionService;
@@ -64,8 +68,10 @@ public class VelocityScheduler implements Scheduler {
    */
   public VelocityScheduler(PluginManager pluginManager) {
     this.pluginManager = pluginManager;
-    this.taskService = Executors.newCachedThreadPool(new ThreadFactoryBuilder().setDaemon(true)
+    this.taskService = new ThreadPoolExecutor(1, MAX_SCHEDULER_POOLED_THREAD_CAP,
-        .setNameFormat("Velocity Task Scheduler - #%d").build());
+        60L, TimeUnit.SECONDS, new LinkedBlockingQueue<>(),
+        new ThreadFactoryBuilder().setDaemon(true)
+            .setNameFormat("Velocity Task Scheduler - #%d").build());
     this.timerExecutionService = Executors
         .newSingleThreadScheduledExecutor(new ThreadFactoryBuilder().setDaemon(true)
             .setNameFormat("Velocity Task Scheduler Timer").build());