Make chain::Access async-optional

[TheBlueMatt]

We'll need to be careful to ensure that any "downstream" messages (ie node_announcement/channel_update) to a pending channel_announcement are held and not processed until the async query resolves. Ideally we can do this without storing the pending messages in the network graph itself, maybe just storing a map from the pending SCIDs to a weak pointer to the future, adding pending to-process messages to the future object itself and making users call back into the graph with the future object.

[TheBlueMatt]

I needed this for a personal project, so I went ahead and did it.