=pro Exclude target directories from find in publish_samples.sh

* It found activator.properties in target directories
fix maven shade docs
=doc #3881 Change link to persistence community plugins
=clu Remove debug log noise of gossip round

* that log entry is not useful
Merge pull request #2025 from akka/wip-3881-persistence-community-patriknw

=doc #3881 Change link to persistence community plugins
=doc Links to activator and some doc improvements
=sam Switch to build.sbt in activator templates
=sam #3887 Public domain license for activator templates
Merge pull request #2024 from akka/wip-publish-samples-target-patriknw

=pro Exclude target directories from find in publish_samples.sh
Merge pull request #2026 from akka/wip-remove-gossip-debug-log-patriknw

=clu Remove debug log noise of gossip round
Merge pull request #2029 from akka/wip-sharding-activator-link-patriknw

=doc Links to activator and some doc improvements
Merge pull request #2030 from akka/wip-activator-build-patriknw

=sam Switch to build.sbt in activator templates
Merge pull request #2031 from akka/wip-3887-activator-cc0-license-patriknw

=sam #3887 Public domain license for activator templates
=act #3623 unsubscribe when actors terminate, instead of `isTerminated`

* Moved to removing actors proactively when they are terminated instead of
  checking `isTerminated` during publish.
* Subscribers which have registered before initializing the unsubscriber
  will be aggregated in a Seq until one is registered and then it will
  take responsibility of unregistering them on termination.
* Initialization of the unsubscriber can only be run once - attempting
  to initialize the event stream with another unsubscriber will fail,
  and init will return false.
* Assumed having an init (mutable) method on the `EventBus` is fine, as
  it has such methods already and @patriknw's comment in the task for
  this.

Review by @rkuhn
=act #3623 addressed review feedback
=act #3623 mark private[akka] methods as internal api, update test
=act #3623 when unsubscribing from stream, unwatch in unsubscriber
=act WIP 3623++ removing `isTerminated` from ActorClassification
=act WIP 3623++ moving around the actor starting to ActorClassification

Which simplifies a lot, no explicit init is needed, and since we start
the actor

TODO there might be a race between eventBus publication and the actor
running the init code hm hm; Will investigate tomorrow
=act #3623 remove racy assertion, not really required in this test