API Proposal: Expose SocketAsyncEventArgs constructor for suppressing the execution context

This constructor should be exposed to give callers the control over not capturing and restoring the execution context per operation.

https://github.com/dotnet/corefx/blob/788690a86f37a9b70907d002a367f57fca8ba423/src/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.cs#L95

```C#
protected SocketAsyncEventArgs(bool flowExecutionContext)
{
}
```

cc @geoffkizer @stephentoub 