gio: make better use of the cached buffer

gio: make better use of the cached buffer

When playing mp3 files from a smb server, we get 64k read requests
that mostly overlap.  Without using the cache to partially satisfy
these, we send these requests straight to the server, resulting in
a lot more network traffic than necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=705415