fix(gql_websocket_link): Fix race condition causing exception and double complete

[james-pellow]

There is a race condition in the subscribe function of the _Client class. If the dispose function returned by the subscribe method is called which calls the releaser() method, and then a CompleteMessage is received on the websocket while the releaser() is awaiting to compose its own CompleteMessage, then releaser() is called twice and the request is double released causing an exception. I've added a flag to avoid a double release.

The second patch simply avoids the situation where a websocket is already closed when an error occurs, causing an already completed exception when completeError() is called on the completer.