Use ZSubscription_t in zephyr_triple

* Change some functions to use `ZSubscription_t`
* Cleanup redundant `zephyr_triple` allocations
* Inline `zephyr_subscribe_failed`
* Replace `find_sub_by_sub` and `find_sub_by_id` with `g_slist_find_custom`
* Rename functions related to `zephyr_triple`

Testing Done:
Compile.

Reviewed at https://reviews.imfreedom.org/r/450/