Support only one GossipSync in BackgroundProcessor

BackgroundProcessor can take an optional P2PGossipSync and an optional
RapidGossipSync, but doing so may be easy to misuse. Each has a
reference to a NetworkGraph, which could be different between the two,
but only one is actually used.

Instead, allow passing one object wrapped in a GossipSync enum. Also,
fix a bug where the NetworkGraph is not persisted on shutdown if only a
RapidGossipSync is given.

Author: jkczyz