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-sharding/src/main/scala/akka/cluster/sharding/Shard.scala

@@ -200,7 +200,7 @@ private[akka] class Shard(
   import context.dispatcher
   val passivateIdleTask = if (settings.passivateIdleEntityAfter > Duration.Zero) {
     val idleInterval = settings.passivateIdleEntityAfter / 2
-    Some(context.system.scheduler.schedule(idleInterval, idleInterval, self, PassivateIdleTick))
+    Some(context.system.scheduler.scheduleWithFixedDelay(idleInterval, idleInterval, self, PassivateIdleTick))
   } else {
     None
   }
@@ -546,7 +546,7 @@ private[akka] class RememberEntityStarter(
 
   val tickTask = {
     val resendInterval = settings.tuningParameters.retryInterval
-    context.system.scheduler.schedule(resendInterval, resendInterval, self, Tick)
+    context.system.scheduler.scheduleWithFixedDelay(resendInterval, resendInterval, self, Tick)
   }
 
   def sendStart(ids: Set[ShardRegion.EntityId]): Unit = {