Introduce a subscribers_list class that exposes 3 methods:
- push_back(s) - adds a new element s to the back of the list
- remove(s) - removes an element s from the list
- for_each(f) - invoke f on each element of the list
- make a subscriber_list store shared_ptr to a subscriber
to allow removing (currently it stores a naked pointer to the object).
subscribers_list allows push_back() and remove() to be called while
another thread (e.g. seastar::async()) is in the middle of for_each().
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
New in v2:
- Simplify subscribers_list::remove() method.
- load_broadcaster: inherit from enable_shared_from_this instead
of async_sharded_service.
