Switch to ValueTask for async methods

[Daniel Cazzulino]

We only have CompleteAsync and RunAsync on IWebSocketPipe, but since the underlying primitives we invoke on either the pipe reader/writer or websocket, all return ValueTask to avoid allocations, we might as well follow suit.

Background: https://devblogs.microsoft.com/dotnet/understanding-the-whys-whats-and-whens-of-valuetask/