Improve channel manager constructor considering RGS

[TheBlueMatt]

No description provided.

[jkczyz]

A couple things that I noticed:

• Disabling P2PGossipSync requires passing null for net_graph_serialized to the constructor.
• But this will start the BackgroundProcessor with GossipSync.none(), which means any NetworkUpdates from failed payments won't be applied to the NetworkGraph

[jkczyz]

Also:

• InvoicePayer won't be initialized without a NetworkGraph, making the previous point irrelevant since there would be no event interception