Add Lightbend's SBR to Akka Cluster, #29085 (#29099)

* change package name to akka.cluster.sbr
* reference.conf has same config paths
* akka.cluster.sbr.SplitBrainResolverProvider instead of com.lightbend.akka.sbr.SplitBrainResolverProvider
* dependency from akka-cluster to akka-coordination, for lease strategy
* move TestLease to akka-coordination and use that in SBR tests
* remove keep-referee strategy
* use keep-majority by default
* review and adjust reference documentation

Co-authored-by: Johan Andrén <johan@markatta.com>
Co-authored-by: Johannes Rudolph <johannes.rudolph@gmail.com>
Co-authored-by: Christopher Batey <christopher.batey@gmail.com>
Co-authored-by: Arnout Engelen <github@bzzt.net>

akka-docs/src/test/scala/docs/coordination/LeaseDocSpec.scala

@@ -0,0 +1,102 @@
+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.coordination
+
+import scala.concurrent.Future
+
+import com.typesafe.config.ConfigFactory
+
+import akka.cluster.Cluster
+import akka.coordination.lease.LeaseSettings
+import akka.coordination.lease.scaladsl.Lease
+import akka.coordination.lease.scaladsl.LeaseProvider
+import akka.testkit.AkkaSpec
+
+//#lease-example
+class SampleLease(settings: LeaseSettings) extends Lease(settings) {
+
+  override def acquire(): Future[Boolean] = {
+    Future.successful(true)
+  }
+
+  override def acquire(leaseLostCallback: Option[Throwable] => Unit): Future[Boolean] = {
+    Future.successful(true)
+  }
+
+  override def release(): Future[Boolean] = {
+    Future.successful(true)
+  }
+
+  override def checkLease(): Boolean = {
+    true
+  }
+}
+//#lease-example
+
+object LeaseDocSpec {
+
+  def config() =
+    ConfigFactory.parseString("""
+      jdocs-lease.lease-class = "jdocs.coordination.LeaseDocTest$SampleLease"
+      #lease-config
+      akka.actor.provider = cluster
+      docs-lease {
+        lease-class = "docs.coordination.SampleLease"
+        heartbeat-timeout = 100s
+        heartbeat-interval = 1s
+        lease-operation-timeout = 1s
+        # Any lease specific configuration
+      }
+      #lease-config
+    """.stripMargin)
+
+  def blackhole(stuff: Any*): Unit = {
+    stuff.toString
+    ()
+  }
+  def doSomethingImportant(leaseLostReason: Option[Throwable]): Unit = {
+    leaseLostReason.map(_.toString)
+    ()
+  }
+}
+
+class LeaseDocSpec extends AkkaSpec(LeaseDocSpec.config) {
+  import LeaseDocSpec._
+
+  "A docs lease" should {
+    "scala lease be loadable from scala" in {
+
+      //#lease-usage
+      val lease = LeaseProvider(system).getLease("<name of the lease>", "docs-lease", "owner")
+      val acquired: Future[Boolean] = lease.acquire()
+      val stillAcquired: Boolean = lease.checkLease()
+      val released: Future[Boolean] = lease.release()
+      //#lease-usage
+
+      //#lost-callback
+      lease.acquire(leaseLostReason => doSomethingImportant(leaseLostReason))
+      //#lost-callback
+
+      //#cluster-owner
+      val owner = Cluster(system).selfAddress.hostPort
+      //#cluster-owner
+
+      // remove compiler warnings
+      blackhole(acquired, stillAcquired, released, owner)
+
+    }
+
+    "java lease be loadable from scala" in {
+      val lease = LeaseProvider(system).getLease("<name of the lease>", "jdocs-lease", "owner")
+      val acquired: Future[Boolean] = lease.acquire()
+      val stillAcquired: Boolean = lease.checkLease()
+      val released: Future[Boolean] = lease.release()
+      lease.acquire(leaseLostReason => doSomethingImportant(leaseLostReason))
+
+      blackhole(acquired, stillAcquired, released)
+    }
+  }
+
+}