Add import for TestProbe

The example for TestProbe doesn't seem to work very well without the TestProbe class imported. :-(
=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>
=doc #15491 java circuit breaker doc update
Merge pull request #15478 from kanerogers/patch-1

Add import for TestProbe
Merge pull request #15492 from gfaerie/master

=doc #15491 java circuit breaker doc update
Update books

Updated release date for "Akka Concurrency" and "Akka in Action" release date estimate, and added references to two books, "Effective Akka" and "Developing an Akka Edge".
Merge pull request #15513 from sslavic/patch-1

Update books info
=ben #15516 correct tell_commandPersist_reply test with right props
=act fixed typo in scaladoc, and => an

Thanks @geoffjentry for spotting this :-)
Merge pull request #15519 from 2m/wip-15516-actor-type-fix

=ben #15516 correct tell_commandPersist_reply test with right props (validation)
=doc #15207 add note about UndeclaredThrowableException when used from Java
Merge pull request #15524 from ujay68/issue-15207-2

=doc #15207 add note about UndeclaredThrowableException when used from Java
+tes #12681 add EchoActor
=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