Group metadata manager & coordinator (#23)

Master Issue: #4 

*Motivation*

Support Kafka group coordinator protocols.

src/main/java/io/streamnative/kop/coordinator/group/DelayedHeartbeat.java

@@ -0,0 +1,58 @@
+/**
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.streamnative.kop.coordinator.group;
+
+import io.streamnative.kop.utils.delayed.DelayedOperation;
+import java.util.Optional;
+
+/**
+ * Delayed heartbeat operations that are added to the purgatory for session timeout checking.
+ * Heartbeats are paused during rebalance.
+ */
+class DelayedHeartbeat extends DelayedOperation {
+
+    private final GroupCoordinator coordinator;
+    private final GroupMetadata group;
+    private final MemberMetadata member;
+    private long heartbeatDeadline;
+
+    DelayedHeartbeat(GroupCoordinator coordinator,
+                     GroupMetadata group,
+                     MemberMetadata member,
+                     long heartbeatDeadline,
+                     long sessionTimeout) {
+        super(sessionTimeout, Optional.of(group.lock()));
+
+        this.coordinator = coordinator;
+        this.group = group;
+        this.member = member;
+        this.heartbeatDeadline = heartbeatDeadline;
+    }
+
+    @Override
+    public void onExpiration() {
+        coordinator.onExpireHeartbeat(group, member, heartbeatDeadline);
+    }
+
+    @Override
+    public void onComplete() {
+        coordinator.onCompleteHeartbeat();
+    }
+
+    @Override
+    public boolean tryComplete() {
+        return coordinator.tryCompleteHeartbeat(group, member, heartbeatDeadline, () -> forceComplete());
+    }
+
+}

src/main/java/io/streamnative/kop/coordinator/group/DelayedJoin.java

@@ -0,0 +1,57 @@
+/**
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.streamnative.kop.coordinator.group;
+
+import io.streamnative.kop.utils.delayed.DelayedOperation;
+import java.util.Optional;
+
+/**
+ * Delayed rebalance operations that are added to the purgatory when group is preparing for rebalance.
+ *
+ * <p>Whenever a join-group request is received, check if all known group members have requested
+ * to re-join the group; if yes, complete this operation to proceed rebalance.
+ *
+ * <p>When the operation has expired, any known members that have not requested to re-join
+ * the group are marked as failed, and complete this operation to proceed rebalance with
+ * the rest of the group.
+ */
+class DelayedJoin extends DelayedOperation {
+
+    final GroupCoordinator coordinator;
+    final GroupMetadata group;
+
+    protected DelayedJoin(GroupCoordinator coordinator,
+                          GroupMetadata group,
+                          long rebalanceTimeout) {
+        super(rebalanceTimeout, Optional.of(group.lock()));
+        this.coordinator = coordinator;
+        this.group = group;
+    }
+
+    @Override
+    public void onExpiration() {
+        coordinator.onExpireJoin();
+    }
+
+    @Override
+    public void onComplete() {
+        coordinator.onCompleteJoin(group);
+    }
+
+    @Override
+    public boolean tryComplete() {
+        return coordinator.tryCompleteJoin(group, () -> forceComplete());
+    }
+
+}

src/main/java/io/streamnative/kop/coordinator/group/GroupConfig.java

@@ -14,11 +14,15 @@
 package io.streamnative.kop.coordinator.group;
 
 import lombok.Data;
+import lombok.Getter;
+import lombok.experimental.Accessors;
 
 /**
  * Group configuration.
  */
 @Data
+@Accessors(fluent = true)
+@Getter
 public class GroupConfig {
 
     private final int groupMinSessionTimeoutMs;