GIOStream: support for unemulated async close()

GIOStream: support for unemulated async close()

Add an implementation of non-thread-emulated async close of a GIOStream
if either of the underlying stream objects support it.

This prevents us from calling close() functions from another thread on
an object that may not be expecting that.  It also allows us to skip the
thread entirely in case our objects support a pure async close.

https://bugzilla.gnome.org/show_bug.cgi?id=741630