Use a simple struct when reading and writing gdbus messages

Use a simple struct when reading and writing gdbus messages

GData*Streams incur significant overhead, and we do not need all of the
functionality that they provide, since we only ever read from/write to
memory when handling message blobs, so it is more performant to use a
simple structure.

https://bugzilla.gnome.org/show_bug.cgi?id=652650