service: address_map: add lookup function that expects address to exist

We will add code that expects id to ip mapping to exist. If it does not
it is better to fail earlier during testing, so add a function that
calls internal error in case there is no mapping.

service/address_map.hh

@@ -298,6 +298,15 @@ public:
         return entry._addr;
     }
 
+    // Same as find() above but expects mapping to exist
+    gms::inet_address get(locator::host_id id) const {
+        try {
+            return find(id).value();
+        } catch (std::bad_optional_access& err) {
+            on_internal_error(rslog, fmt::format("No ip address for {} when one is expected", id));
+        }
+    }
+
     // Find an id with a given mapping.
     //
     // If a mapping is expiring, the last access timestamp is updated automatically.