Share bookkeeping data across controllers (#2531)

* Use akka distributed map to store the shared state

* Join seed nodes in the load balancer service

* Add optional auto-down-unreachable-after

* Local bookkeeping is used by default

* Update documentation

tests/build.gradle

@@ -48,6 +48,7 @@ dependencies {
     compile 'junit:junit:4.11'
     compile 'com.jayway.restassured:rest-assured:2.6.0'
     compile 'org.scalatest:scalatest_2.11:3.0.1'
+    compile 'com.typesafe.akka:akka-testkit_2.11:2.5.4'
     compile 'com.google.code.gson:gson:2.3.1'
     compile 'org.scalamock:scalamock-scalatest-support_2.11:3.4.2'
     compile 'com.typesafe.akka:akka-testkit_2.11:2.4.16'