Bluetooth: gatt: Fix DB corruption when adding/removing service

Fix an issue in the gatt_register routine where removing a service and
adding it again would cause the database to have non-ascending
orderdered handles numbers and lead to an incomplete service discovery.

Fix: Go through the database and look for a place where to insert
the new service.

Signed-off-by: Arthur Crepin-Leblond <arthur.crepin@proglove.de>
Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>