Merge branch 'amiranda/74-server_id-should-not-be-optional-in-adhoc_storage' into 'main'

Resolve "server_id should not be optional in adhoc_storage"

This MR makes the `server_id` member in adhoc_storage explicit. The rationale
is that constructors (for the C++ API) or `.*_create()` functions (for the C API)
will most of the time be called from internal code, since client code will rely
mostly on `ADM_register_.*()` to obtain handlers.

In this scenario, there's no possibility for the `server_id` "to not having been 
assigned yet", which is why it was `optional` in the first place.

Also, this MR renames the `user_id` to `name`, and the `server_id` to `id`.

Closes #74 #75

See merge request !51