Add IGD discovery phase

Current behaviour: It is impossible to determine if the IGD is unavailable or simply not yet discovered.

We need to know whether to wait for the mappings or not.

Proposed behaviour: When the protocols initiate the search for an IGD, the IGD discovery phase begins. During this phase, all requested mappings are marked as pending. If an IGD is found, the pending mappings will be requested from the router. If the phase ends due to a timeout, the pending mappings will be marked as failed. Any new mapping request after this phase will be automatically marked as failed.