* 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
* Add CopyrightHeader support for sbt-boilerplate plugin.
* Add CopyrightHeader support for `*.proto` files.
* Add regex match for both `–` and `-` for CopyrightHeader.
* Add CopyrightHeader support for sbt build files.
* Update copyright from 2018 to 2019.
* backport of the timers from Akka Typed, #16742
* also fixed a small bug in FSM timers, which could result in that
a timer from a previous incarnation was let through to new
incarnation after restart
* no more need for the complicated "how to" section in docs of
how to schedule periodic messages
