[XrdSsiPb] Updates submodule to v0.23

Adds better cleanup of Requests. The Send() method is synchronous as
before, for Metadata requests. The new SendAsync() method is for Data
and Stream requests, where the Response is returned asynchronously.

The framework now provides a DataFuture object for the asynch case.
After the Request has been sent, call DataFuture::wait() to clean up the
Request object once all data has been received.