Add information about async tasks to CrashReports. Addresses BUKKIT-2491

Async tasks are notorious for causing CMEs and corrupted data when
accessing the API. This change makes a linked list to track recent tasks
that may no longer be running. It is accessed via the toString method on
the scheduler. This behavior is not guaranteed, but it is accessible as
such currently.

Although toString is located in the scheduler, its contract does not
guarantee an accurate or up to date call when accessed from a second
thread.

src/main/java/org/bukkit/craftbukkit/scheduler/CraftAsyncDebugger.java

@@ -0,0 +1,37 @@
+package org.bukkit.craftbukkit.scheduler;
+
+import org.bukkit.plugin.Plugin;
+
+
+class CraftAsyncDebugger {
+    private CraftAsyncDebugger next = null;
+    private final int expiry;
+    private final Plugin plugin;
+    private final Class<? extends Runnable> clazz;
+
+    CraftAsyncDebugger(final int expiry, final  Plugin plugin, final Class<? extends Runnable> clazz) {
+        this.expiry = expiry;
+        this.plugin = plugin;
+        this.clazz = clazz;
+
+    }
+
+    final CraftAsyncDebugger getNextHead(final int time) {
+        CraftAsyncDebugger next, current = this;
+        while (time > current.expiry && (next = current.next) != null) {
+            current = next;
+        }
+        return current;
+    }
+
+    final CraftAsyncDebugger setNext(final CraftAsyncDebugger next) {
+        return this.next = next;
+    }
+
+    StringBuilder debugTo(final StringBuilder string) {
+        for (CraftAsyncDebugger next = this; next != null; next = next.next) {
+            string.append(plugin.getDescription().getName()).append(':').append(clazz.getName()).append('@').append(expiry).append(',');
+        }
+        return string;
+    }
+}