Add a create function for rend_data_t object

Ground works for fixing #15816. This adds the rend_data_create() function in
order to have a single place where we initialize that data structure.

Furthermore, an array of descriptor IDs is added (one per replica) so we can
keep a copy of the current id in the object. It will be used to purge the
last hid serv request cache using those descriptor IDs. When they change,
they will be replaced and the old ones will be purged from the cache.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>