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/java/jdocs/coordination/LeaseDocTest.java

@@ -0,0 +1,105 @@
+/*
+ * Copyright (C) 2019-2020 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package jdocs.coordination;
+
+import akka.actor.ActorSystem;
+import akka.coordination.lease.LeaseSettings;
+import akka.coordination.lease.javadsl.Lease;
+import akka.coordination.lease.javadsl.LeaseProvider;
+import akka.testkit.javadsl.TestKit;
+import docs.coordination.LeaseDocSpec;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.scalatestplus.junit.JUnitSuite;
+
+import java.util.Optional;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionStage;
+import java.util.function.Consumer;
+
+@SuppressWarnings("unused")
+public class LeaseDocTest extends JUnitSuite {
+  // #lease-example
+  static class SampleLease extends Lease {
+
+    private LeaseSettings settings;
+
+    public SampleLease(LeaseSettings settings) {
+      this.settings = settings;
+    }
+
+    @Override
+    public LeaseSettings getSettings() {
+      return settings;
+    }
+
+    @Override
+    public CompletionStage<Boolean> acquire() {
+      return CompletableFuture.completedFuture(true);
+    }
+
+    @Override
+    public CompletionStage<Boolean> acquire(Consumer<Optional<Throwable>> leaseLostCallback) {
+      return CompletableFuture.completedFuture(true);
+    }
+
+    @Override
+    public CompletionStage<Boolean> release() {
+      return CompletableFuture.completedFuture(true);
+    }
+
+    @Override
+    public boolean checkLease() {
+      return true;
+    }
+  }
+  // #lease-example
+
+  private static ActorSystem system;
+
+  @BeforeClass
+  public static void setup() {
+    system = ActorSystem.create("LeaseDocTest", LeaseDocSpec.config());
+  }
+
+  @AfterClass
+  public static void teardown() {
+    TestKit.shutdownActorSystem(system);
+    system = null;
+  }
+
+  private void doSomethingImportant(Optional<Throwable> leaseLostReason) {}
+
+  @Test
+  public void javaLeaseBeLoadableFromJava() {
+    // #lease-usage
+    Lease lease =
+        LeaseProvider.get(system).getLease("<name of the lease>", "jdocs-lease", "<owner name>");
+    CompletionStage<Boolean> acquired = lease.acquire();
+    boolean stillAcquired = lease.checkLease();
+    CompletionStage<Boolean> released = lease.release();
+    // #lease-usage
+
+    // #lost-callback
+    lease.acquire(this::doSomethingImportant);
+    // #lost-callback
+
+    // #cluster-owner
+    // String owner = Cluster.get(system).selfAddress().hostPort();
+    // #cluster-owner
+
+  }
+
+  @Test
+  public void scalaLeaseBeLoadableFromJava() {
+    Lease lease =
+        LeaseProvider.get(system).getLease("<name of the lease>", "docs-lease", "<owner name>");
+    CompletionStage<Boolean> acquired = lease.acquire();
+    boolean stillAcquired = lease.checkLease();
+    CompletionStage<Boolean> released = lease.release();
+    lease.acquire(this::doSomethingImportant);
+  }
+}