+per,+str #15035 Reactive-stream producers for persistent messages written by akka-persistence processors

(cherry picked from commit 5dc62400e4f2f9d7afe5efe93fc8bef5b9d226c0)

Conflicts:
	akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala
	project/AkkaBuild.scala
=per #15429 Rewrite persistence documentation and samples for 2.3.4 changes

(cherry picked from commit 02351e32f110a8c4a249f0f3f84bae5898d1a836)

Conflicts:
	akka-samples/akka-sample-persistence-java-lambda/tutorial/index.html
	akka-samples/akka-sample-persistence-java/tutorial/index.html
	akka-samples/akka-sample-persistence-scala/build.sbt
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/ConversationRecoveryExample.scala
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/PersistentActorExample.scala
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/ProcessorChannelExample.scala
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/ProcessorChannelRemoteExample.scala
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/SnapshotExample.scala
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/StreamExample.scala
	akka-samples/akka-sample-persistence-scala/tutorial/index.html
Merge pull request #15444 from akka/wip-15429-persistence-docs-master-patriknw

=per #15429 Rewrite persistence documentation and samples (for validation)