scheduleWithFixedDelay vs scheduleAtFixedRate, #26910

* previous `schedule` method is trying to maintain a fixed average frequency
  over time, but that can result in undesired bursts of scheduled tasks after a long
  GC or if the JVM process has been suspended, same with all other periodic
  scheduled message sending via various Timer APIs
* most of the time "fixed delay" is more desirable
* we can't just change because it's too big behavioral change and some might
  depend on previous behavior
* deprecate the old `schedule` and introduce new `scheduleWithFixedDelay`
  and `scheduleAtFixedRate`, when fixing the deprecation warning users should
  make a concious decision of which behavior to use (scheduleWithFixedDelay in
  most cases)

* Streams
* SchedulerSpec
  * test both fixed delay and fixed rate
* TimerSpec
* FSM and PersistentFSM
* mima
* runnable as second parameter list, also in typed.Scheduler
* IllegalStateException vs SchedulerException
* deprecated annotations
* api and reference docs, all places
* migration guide

akka-cluster-metrics/src/main/scala/akka/cluster/metrics/ClusterMetricsCollector.scala

@@ -161,12 +161,16 @@ private[metrics] class ClusterMetricsCollector extends Actor with ActorLogging {
    * Start periodic gossip to random nodes in cluster
    */
   val gossipTask =
-    scheduler.schedule(PeriodicTasksInitialDelay max CollectorGossipInterval, CollectorGossipInterval, self, GossipTick)
+    scheduler.scheduleWithFixedDelay(
+      PeriodicTasksInitialDelay max CollectorGossipInterval,
+      CollectorGossipInterval,
+      self,
+      GossipTick)
 
   /**
    * Start periodic metrics collection
    */
-  val sampleTask = scheduler.schedule(
+  val sampleTask = scheduler.scheduleWithFixedDelay(
     PeriodicTasksInitialDelay max CollectorSampleInterval,
     CollectorSampleInterval,
     self,