+cam #3885 Add the ability to use an external camel context

(cherry picked from commit c71987f4a7da496df7ccbcbb206aa54c841da3fe)
=pro #3913 Enable binary compatibility check for 2.3.x
Merge pull request #2059 from akka/wip-3913-mima-2.3-patriknw

=pro #3913 Enable binary compatibility check for 2.3.x
=act, rem #2556 Optimize subscriptions of AddressTerminated

* unsubscribe in eventStream is too slow when using many actors
  that are watching remote actors, and becomes a problem for example
  when shutting down such a system
* Previous eventStream solution:
  Stopping 20000 actors took 50355 ms
* This solution:
  Stopping 20000 actors took 764 ms

(cherry picked from commit 67925cb94eab0c86eecddb1be143477310ddb16c)
Merge pull request #2065 from akka/wip-2556-optimize-AddressTerminated-subscriptions-2.3-patriknw

=act, rem #2556 Optimize subscriptions of AddressTerminated (backport)
+con #3843 Add ClusterSingletonProxy

(cherry picked from commit ee01a8dffe79b249fb6799e08c8ed677de1b585a)
=sam #3843 Use ClusterSingletonProxy in cluster samples

(cherry picked from commit ad184058777740089ca363e6ed96cfbb3155d5fd)
+act #3911 Adding Java Lambda compatibility for Supervisor Strategy
=doc clean up what-is-akka.rst and switch to www.typesafe.com

the latter is necessary because of broken DNS requirements which make
apex domains brittle (since they must resolve to an A record with a
single IP)
fix project name of akka-sample-fsm-scala
Merge pull request #2068 from akka/wip-3911-add-java8-directive-builder-ban

+act #3911 Adding Java Lambda compatibility for Supervisor Strategy
+sam #3925 Adding Supervision for Java with Lambda Support Activator template
Merge pull request #2071 from akka/wip-3843-singleton-proxy-backport-patriknw

+con #3843 Add ClusterSingletonProxy (backport)
+pro #3910 Make the build package all Activator templates
Merge pull request #2069 from akka/wip-3925-supervision-lambda-activator-ban

+sam #3925 Adding Supervision for Java with Lambda Support Activator
Merge pull request #2074 from akka/wip-fsm-sample-project-name-∂π

fix project name of akka-sample-fsm-scala (release-2.3)
Merge pull request #2077 from akka/wip-3910-zip-up-activator-templates-ban

+pro #3910 Make the build package all Activator templates
+pro #3907 fix OSGi Scala version constraint
Merge pull request #2085 from akka/wip-2.3-3907-OSGi-ScalaVersion-∂π

+pro #3907 fix OSGi Scala version constraint
=per #3915 Make become work during recovery for EventsourcedProcessor et.c.