client: Use sync methods instead of flushing dbus

g_dbus_connection_flush_sync causes troubles in certain cases when
using multiple threads. The problem occurs when there are ongoing
gvfs_metadata_call_set_sync calls. It blocks threads e.g. when
moving files over Nautilus. Nautilus freezes sometimes due to it.
Use sync methods instead of flushing dbus. I don't see any significant
slowdown when using sync methods instead of the flushing according
to my testing. It fixes hangs in Nautilus and moving in Nautilus is
almost instant again.

https://bugzilla.gnome.org/show_bug.cgi?id=757747