trivial: hif: Remove a #if that can never be false
Make pk_backend_repo_list_changed() threadsafe
Add pk_is_thread_default() to detect possible threading problems
Fix a hard-to-debug crash when cancelling a task that has never been run

Calling g_dbus_method_invocation_return_value() twice for the same
GDBusMethodInvocation is a bad idea that manifests in a backtrace like:

 > g_source_callback_unref (cb_data) at gmain.c:1532
 > g_source_destroy_internal (source, context, have_lock=1) at gmain.c:1178
 > g_main_context_dispatch (context) at gmain.c:3134

 > (gdb) p g_source_get_name(source)
 > $1 = '[gio] call_in_idle_cb'