Add import for TestProbe

The example for TestProbe doesn't seem to work very well without the TestProbe class imported. :-(
=doc #15491 java circuit breaker doc update
=doc fixed minor typo
Merge pull request #15504 from tsabirgaliev/patch-2

=doc fixed minor typo
=doc fix typo found in #15504 in the java version of docs
Merge pull request #15505 from ktoso/fix-typo-ktoso

=doc fix typo found in #15504 in the java version of docs
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
+act #15502: Pluggable DNS resolution in akka-io