[th/config-match-lookup]

nm_config_data_get_connection_default() gets called a lot.

Mostly by nm_device_get_route_metric().

In an example profiling run activating a 100 devices it thus shows up to consume 13.3% of the CPU time. That's mainly due to the overhead of g_key_file_get_string() (interestingly, it is due to glib_gettext() to construct the GError message). Anyway, it sucks.

Add our own lookup index for these values and use binary search.