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/CraftCrashReport.java

@@ -28,7 +28,7 @@ public class CraftCrashReport implements Callable {
             for (Map.Entry<Thread, ? extends Object[]> entry : Thread.getAllStackTraces().entrySet()) {
                 value.append(' ').append(entry.getKey().getState().name()).append(' ').append(entry.getKey().getName()).append(": ").append(Arrays.toString(entry.getValue())).append(',');
             }
-            value.append('}');
+            value.append("}\n   ").append(Bukkit.getScheduler().toString());
         } catch (Throwable t) {
             value.append("\n   Failed to handle CraftCrashReport:\n");
             PrintWriter writer = new PrintWriter(value);