Abstract the underlying poll

Author: afshin

packages/polling/src/socketstream.ts

@@ -75,9 +75,9 @@ export class SocketStream<T, U> extends Stream<T, U> implements IDisposable {
   protected socket: WebSocket | null = null;
 
   /**
-   * The poll instance that mediates the web socket lifecycle.
+   * A handle to the socket subscription to dispose when necessary.
    */
-  protected readonly subscription: Poll;
+  protected readonly subscription: IDisposable;
 
   /**
    * Open a web socket and subscribe to its updates.