Rename event-handlers to loggers, see #2979

* Rename config akka.event-handlers to akka.loggers
* Rename config akka.event-handler-startup-timeout to
  akka.logger-startup-timeout
* Rename JulEventHandler to JavaLogger
* Rename Slf4jEventHandler to Slf4jLogger
* Change all places in tests and docs
* Deprecation, old still works, but with warnings
* Migration guide
* Test for the deprecated event-handler config