NeighborListCache: don't build neighbor list during initialization

[chenlijun99]

Given n radios, neighbor list construction has O(n^2) complexity. If, during initialization, when each radio is added, the neighbor list is reconstructed, we end up having a simulation initialization with O(n^3) complexity. Sidenote: In one of our tests, without this patch, when running a WiFi simulation of 5000 nodes, it took ~40 minutes to just initialize the simulation.

[levy]

Thanks for submitting this PR!