[service-bus] init() refactor to propagate abortSignal support. (Azure#10578)

Uniting all the "link" related entities (mgmt link, receiver, sender).

This PR unites all the links so they use the same code to open and close 
the underlying link. As part of this a few nice refactors came in:

1. All link related classes, including the mgmt link, now use the same
init() code (which also means they can, if passed, handle abortSignal's
when running)
2. open/close state has been moved into LinkEntity.
3. Boundaries between LinkEntity and the child classes is more clear.

Author: richardpark-msft

sdk/servicebus/service-bus/src/core/batchingReceiver.ts

@@ -13,12 +13,7 @@ import {
   Session
 } from "rhea-promise";
 import { InternalReceiveMode, ServiceBusMessageImpl } from "../serviceBusMessage";
-import {
-  MessageReceiver,
-  OnAmqpEventAsPromise,
-  ReceiveOptions,
-  ReceiverType
-} from "./messageReceiver";
+import { MessageReceiver, OnAmqpEventAsPromise, ReceiveOptions } from "./messageReceiver";
 import { ConnectionContext } from "../connectionContext";
 import { throwErrorIfConnectionClosed } from "../util/errors";
 import { AbortSignalLike } from "@azure/abort-controller";
@@ -41,7 +36,7 @@ export class BatchingReceiver extends MessageReceiver {
    * @param {ReceiveOptions} [options]  Options for how you'd like to connect.
    */
   constructor(context: ConnectionContext, protected _entityPath: string, options?: ReceiveOptions) {
-    super(context, _entityPath, ReceiverType.batching, options);
+    super(context, _entityPath, "br", options);
 
     this._batchingReceiverLite = new BatchingReceiverLite(
       context,
@@ -69,7 +64,7 @@ export class BatchingReceiver extends MessageReceiver {
           throw lastError;
         }
 
-        return this._receiver;
+        return this.link;
       },
       this.receiveMode
     );
@@ -87,8 +82,7 @@ export class BatchingReceiver extends MessageReceiver {
    * @returns {Promise<void>} Promise<void>.
    */
   async onDetached(connectionError?: AmqpError | Error): Promise<void> {
-    // Clears the token renewal timer. Closes the link and its session if they are open.
-    await this._closeLink(this._receiver);
+    await this.closeLink("linkonly");
 
     if (connectionError == null) {
       connectionError = new Error(