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-docs/src/test/scala/docs/actor/TimerDocSpec.scala

@@ -24,7 +24,7 @@ object TimerDocSpec {
     def receive = {
       case FirstTick =>
         // do something useful here
-        timers.startPeriodicTimer(TickKey, Tick, 1.second)
+        timers.startTimerWithFixedDelay(TickKey, Tick, 1.second)
       case Tick =>
       // do something useful here
     }