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 (closed) #75 (closed)