s3-mdssvc: factor out Tracker backend logic

s3-mdssvc: factor out Tracker backend logic

This moves all Tracker backend logic into a modularized component.

This should not result in any change in behaviour, it just paves the way
for adding additional backends. Currently the only available backend is
Gnome Tracker.

slq_destroy_send/recv is not needed anymore as the problem is solved now by
correctly checking if an async Tracker request was cancelled and we got
G_IO_ERROR_CANCELLED in tracker_con_cb() or tracker_query_cb() and avoid using
user_data in that the case.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>