Schedule cluster tasks with more accurate, see #2114

* Use scheduler with more accurate settings
* New FixedRateTask that compensates for inaccuracy

akka-cluster/src/main/scala/akka/cluster/FixedRateTask.scala

@@ -0,0 +1,54 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.cluster
+
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicLong
+
+import akka.actor.Scheduler
+import akka.util.Duration
+
+/**
+ * INTERNAL API
+ */
+private[akka] object FixedRateTask {
+  def apply(scheduler: Scheduler, initalDelay: Duration, delay: Duration)(f: ⇒ Unit): FixedRateTask = {
+    new FixedRateTask(scheduler, initalDelay, delay, new Runnable { def run(): Unit = f })
+  }
+}
+
+/**
+ * INTERNAL API
+ *
+ * Task to be scheduled periodically at a fixed rate, compensating, on average,
+ * for inaccuracy in scheduler. It will start when constructed, using the
+ * initialDelay.
+ */
+private[akka] class FixedRateTask(scheduler: Scheduler, initalDelay: Duration, delay: Duration, task: Runnable) extends Runnable {
+
+  private val delayMillis = delay.toMillis
+  private val minDelayMillis = 1L
+  private val cancelled = new AtomicBoolean(false)
+  private val counter = new AtomicLong(0L)
+  private val startTime = System.currentTimeMillis + initalDelay.toMillis
+  scheduler.scheduleOnce(initalDelay, this)
+
+  def cancel(): Unit = cancelled.set(true)
+
+  override final def run(): Unit = if (!cancelled.get) try {
+    task.run()
+  } finally if (!cancelled.get) {
+    val nextTime = startTime + delayMillis * counter.incrementAndGet
+    val nextDelayMillis = nextTime - System.currentTimeMillis
+    val nextDelay = Duration(
+      (if (nextDelayMillis <= minDelayMillis) minDelayMillis else nextDelayMillis),
+      TimeUnit.MILLISECONDS)
+    try {
+      scheduler.scheduleOnce(nextDelay, this)
+    } catch { case e: IllegalStateException ⇒ /* will happen when scheduler is closed, nothing wrong */ }
+  }
+
+}