feat: Create Stream writer v2 - starting with core logics (#867)

* Add a new StreamWriterV2.

Compared to existing StreamWriter, its locking mechanism is much
simpler.

* Stop using Java8 features as we still need to support Java7

* Do not hold lock while sending requests, and some minor refactoring.

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1beta2/StreamConnection.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     https://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 com.google.cloud.bigquery.storage.v1beta2;
+
+import com.google.api.gax.rpc.BidiStreamingCallable;
+import com.google.api.gax.rpc.ClientStream;
+import com.google.api.gax.rpc.ResponseObserver;
+import com.google.api.gax.rpc.StreamController;
+import io.grpc.Status;
+import io.grpc.Status.Code;
+import io.grpc.StatusRuntimeException;
+
+/**
+ * StreamConnection is responsible for writing requests to a GRPC bidirecional connection.
+ *
+ * <p>StreamWriter creates a connection. Two callback functions are necessary: request_callback and
+ * done_callback. Request callback is used for every request, and done callback is used to notify
+ * the user that the connection is closed and no more callbacks will be received from this
+ * connection.
+ *
+ * <p>The stream writer will accept all the requests without flow control, and makes the callbacks
+ * in receiving order.
+ *
+ * <p>It's user's responsibility to do the flow control and maintain the lifetime of the requests.
+ */
+public class StreamConnection {
+  private BidiStreamingCallable<AppendRowsRequest, AppendRowsResponse> bidiStreamingCallable;
+  private ClientStream<AppendRowsRequest> clientStream;
+
+  private RequestCallback requestCallback;
+  private DoneCallback doneCallback;
+
+  public StreamConnection(
+      BigQueryWriteClient client, RequestCallback requestCallback, DoneCallback doneCallback) {
+    this.requestCallback = requestCallback;
+    this.doneCallback = doneCallback;
+
+    bidiStreamingCallable = client.appendRowsCallable();
+    clientStream =
+        bidiStreamingCallable.splitCall(
+            new ResponseObserver<AppendRowsResponse>() {
+
+              @Override
+              public void onStart(StreamController controller) {
+                // no-op
+              }
+
+              @Override
+              public void onResponse(AppendRowsResponse response) {
+                StreamConnection.this.requestCallback.run(response);
+              }
+
+              @Override
+              public void onError(Throwable t) {
+                StreamConnection.this.doneCallback.run(t);
+              }
+
+              @Override
+              public void onComplete() {
+                StreamConnection.this.doneCallback.run(
+                    new StatusRuntimeException(
+                        Status.fromCode(Code.CANCELLED)
+                            .withDescription("Stream is closed by user.")));
+              }
+            });
+  }
+
+  /**
+   * Sends a request to the bi-directional stream connection.
+   *
+   * @param request request to send.
+   */
+  public void send(AppendRowsRequest request) {
+    clientStream.send(request);
+  }
+
+  /** Close the bi-directional stream connection. */
+  public void close() {
+    clientStream.closeSend();
+  }
+
+  /** Invoked when a response is received from the server. */
+  public static interface RequestCallback {
+    public void run(AppendRowsResponse response);
+  }
+
+  /** Invoked when server closes the connection. */
+  public static interface DoneCallback {
+    public void run(Throwable finalStatus);
+  }
+}