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)

examples/c/ADM_cancel_transfer.c

@@ -68,11 +68,11 @@ main(int argc, char* argv[]) {
             100, false);
     assert(ctx);
 
-    const char* user_id = "adhoc_storage_42";
+    const char* name = "adhoc_storage_42";
 
     ADM_storage_t adhoc_storage;
     ADM_return_t ret =
-            ADM_register_adhoc_storage(server, user_id, ctx, &adhoc_storage);
+            ADM_register_adhoc_storage(server, name, ctx, &adhoc_storage);
 
     if(ret != ADM_SUCCESS) {
         fprintf(stdout, "ADM_register_adhoc_storage() remote procedure not "