The new AsyncPaymentTriggerer singleton actor spawns only a single PeerReadyNotifier actor for each peer that is watched. This prevents multiple actors from redundantly polling for when the same peer reconnects.

Each Watcher for a given peer triggers a specific async payment (managed by a NodeRelay actor) and has a unique timeout (in block height). The NodeRelay actor will receive an AsyncPaymentTriggered when the target peer reconnects, or AsyncPaymentTimeout if the timeout block height is reached.

This PR requires a change to allow PeerReadyNotifier to have an infinite timeout so that AsyncPaymentTriggerer can instead manage multiple timeouts for a given peer.

The AsyncPaymentTriggerer singleton must be created before the Relayer actor is created, but requires a reference to the singleton Switchboard actor to spawn PeerReadyNotifier actors. Because the swtichboard is indirectly instantiated with the Relayer reference (via PeerFactory and ChannelFactory), the AsyncPaymentTriggerer actor must be further initialized by sending a Start message with the Switchboard actor reference before being used.

In the future the AsyncPaymentTriggerer actor can be extended to watch for onion messages from async payment receivers, instead of only watching for local peer re-connections. In the onion message case, the paymentHash will be used to identify which Watcher to trigger.