In this commit, we ensure that any topology update is forced to go via
the `handleTopologySubscriptions` handler so that client subscriptions
and updates are handled correctly and in the correct order.
This removes a bug that could result from a client missing a
notification about a channel being closed if the client is subscribed
and shortly after, `PruneGraph` is called which would notify all
subscribed clients and possibly do so before the client subscription has
actually been persisted.
We plan to later on add an option for a remote graph source which will
be managed from the ChannelGraph. In such a set-up, a node would rely on
the remote graph source for graph updates instead of from gossip sync.
In this scenario, however, our topology subscription logic should still
notify clients of all updates and so it makes more sense to have the
logic as part of the ChannelGraph so that we can send updates we receive
from the remote graph.