Reset network graph upon changing gossip source

[tnull]

We already reset the last RGS syncing timestamp on switching gossip sources, but we should also reset the network graph data itself, as otherwise we might be unable to apply RGS updates to a previously-P2PGossip synced graph.

[TheBlueMatt]

Hmm, this shouldn't be the case, it should apply cleanly. Is there a specific case where it didn't? cc @arik-so