=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>
+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