BUG/MINOR: resolvers: mark servers with name-resolution as non purgeable

When a server is configured with name-resolution, resolvers objects are
created with reference to this server. Thus the server is marked as non
purgeable to prevent its removal at runtime.

This does not need to be backport.

src/resolvers.c

@@ -2470,6 +2470,8 @@ static int resolvers_finalize_config(void)
 				err_code |= (ERR_ALERT|ERR_ABORT);
 				continue;
 			}
+
+			srv->flags |= SRV_F_NON_PURGEABLE;
 		}
 	}