From now on the `call_subscription` concept gets
deprecated, and instead of it the `media_subscriptions`
concept gets applied.
Benefits of this change is:
- ability to subscribe one-to-multiple medias (different monologues)
- media level manipulations, without affecting whole SDP session
- no need to use medias offset, to detect proper subscription's media
- there is no need of particular medias order, they can be
subscribed to each other in any possible way
(even though RFC still requires to always have proper ordering)
Deprecated objects:
- `struct call_subscription`
- `GQueue subscriptions`
- `GQueue subscribers`
- `GHashTable * subscriptions_ht`
- `GHashTable * subscribers_ht`
Deprecated functionality:
- `__unsubscribe_one()`
- `__unsubscribe_all_offer_answer_subscribers()`
- `__unsubscribe_from_all()`
- `__subscribe_offer_answer_both_ways()`
- `__add_subscription()`
- `__unsubscribe_one_link()`
- `call_get_call_subscription()`
- `call_subscriptions_clear()`
- `call_subscriptions_free()`
Offtopic: additionally this commit adds helper func:
- `call_media_subscribed_to_monologue()`
Change-Id: Ifb44f7a1ba5b483b1472882b1b8d06444dba1727