=act #15488 Removed guard around _logLevel

This patch removes a lock around the getter of _logLevel. The setter is
still guarded for consistency. Note that out of thin-air reads, while
not excluded under the current JMM, are forbidden by current hardware.

Signed-off-by: Duarte Nunes <duarte.m.nunes@gmail.com>
=sam #12702 Create FsmSimpleRedelivery sample

This one is created instead of the batching pattern of #12702, as suggested by @ktoso in #12702's discussion.
Merge pull request #15368 from michalrus/fsm-simple-redelivery

=sam #12702 Create FsmSimpleRedelivery sample
+per #13815 akka-persistence-tck based on @krasserm's work

Original here: https://github.com/krasserm/akka-persistence-testkit

New features:
* merged martin's tests
* usable from java (junit 4)
* simple bench test, which helps checking if ordering is perserved under
  bigger workloads and simple perf checking
* does NOT include tests for already deprecated features (deleteMessages)
* docs

Resolves #13815

Conflicts:
	project/AkkaBuild.scala
Merge pull request #15527 from ktoso/port-persistence-tck-ktoso

+per #13815 akka-persistence-tck based on @krasserm's work
+doc #15537 add Typed router pattern

Conflicts:
	akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala
Merge pull request #15541 from ktoso/port-doc-typed-router-ktoso

+doc #15537 add Typed router pattern (for validation)
Merge pull request #15481 from duarten/master

Removed guard around _logLevel
=doc #15543 link to persistence in java docs should point to java persistence
Merge pull request #15549 from ktoso/port-fix-java-persistence-link-ktoso

=doc #15543 java docs should link to java persistence (for Validation)
=act #15488 add @volatile logLevel, and benchmark

Summary:
* made `logLevel` @volatile
* Perf benchmark and results are attached.

Please review @viktorklang
Merge pull request #15553 from ktoso/port-volatileLogLevel-ktoso

=act #15488 add @volatile logLevel, and benchmark (for validation)
=act #13970 Send transistion on goto(CurrentState) in FSM

When in `A`:
* `goto(A)` will trigger `onTransition(A -> A)`
* `stay()` will NOT trigger `onTransition`

Includes:
* migration guide
* docs updates
* test
Merge pull request #15561 from ktoso/master

=act #13970 Send transistion on goto(CurrentState) in FSM
Merge remote-tracking branch 'upstream/master'
+act #15529 selection scheduling update based on CR feedback