Configuration knob to disable client-id lookup

While investigating a performance issue for a customer issue, @tmark came up with an optimization idea to optionally disable client-id lookup.

Kea does two lease lookups: client-id (if client sent client-id) and if a lease is not found, another one by MAC address. If admin doesn't care about client-id, the first one could be disabled.

This ticket's goal is to make this lookup optional. Obviously it should be enabled by default, but there should be a knob to disable it.

Instead of adding a new config knob, we should expand the meaning of existing match-client-id parameter.