LightArrayRevolverScheduler, see #2904

- based on a wheel (AtomicReferenceArray) from which atomic
  single-linked lists dangle
- no locks
- deterministic tests due to overridable time source
- also bring docs up to date

akka-actor/src/main/java/akka/actor/AbstractScheduler.java

@@ -0,0 +1,53 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.actor;
+
+import scala.concurrent.ExecutionContext;
+import scala.concurrent.duration.FiniteDuration;
+
+//#scheduler
+/**
+ * An Akka scheduler service. This one needs one special behavior: if
+ * Closeable, it MUST execute all outstanding tasks upon .close() in order
+ * to properly shutdown all dispatchers.
+ *
+ * Furthermore, this timer service MUST throw IllegalStateException if it
+ * cannot schedule a task. Once scheduled, the task MUST be executed. If
+ * executed upon close(), the task may execute before its timeout.
+ *
+ * Scheduler implementation are loaded reflectively at ActorSystem start-up
+ * with the following constructor arguments:
+ *  1) the system’s com.typesafe.config.Config (from system.settings.config)
+ *  2) a akka.event.LoggingAdapter
+ *  3) a java.util.concurrent.ThreadFactory
+ */
+public abstract class AbstractScheduler extends AbstractSchedulerBase {
+
+  /**
+   * Schedules a function to be run repeatedly with an initial delay and
+   * a frequency. E.g. if you would like the function to be run after 2
+   * seconds and thereafter every 100ms you would set delay = Duration(2,
+   * TimeUnit.SECONDS) and interval = Duration(100, TimeUnit.MILLISECONDS)
+   */
+  @Override
+  public abstract Cancellable schedule(FiniteDuration initialDelay,
+      FiniteDuration interval, Runnable runnable, ExecutionContext executor);
+
+  /**
+   * Schedules a Runnable to be run once with a delay, i.e. a time period that
+   * has to pass before the runnable is executed.
+   */
+  @Override
+  public abstract Cancellable scheduleOnce(FiniteDuration delay, Runnable runnable,
+      ExecutionContext executor);
+
+  /**
+   * The maximum supported task frequency of this scheduler, i.e. the inverse
+   * of the minimum time interval between executions of a recurring task, in Hz.
+   */
+  @Override
+  public abstract double maxFrequency();
+}
+//#scheduler