In this commit, we add a new method to allow external sub-systems to
gain an intent to receive notifications once an on-chain event happens.
This will be used in place of the old channel signals directly on the
channel state machine object in a series of follow up commits.
In this commit, we modify the construction of the channel arbitrator to
accept a pointer to an event stream from the chain watcher that’s been
assigned to that channel. As a result, we no longer need a fresh
unilateral close signal, as the one we get from the chain watcher will
*always* be up to date.
For each active channel, we’ll now create a chainWatcher instance that
will be around until the channel is fully closed on chain.
In this commit, we add the ChainArbitrator struct. The ChainArbitrator
is a special sub-system that will oversee the on-chain resolution of
all active channels, and also channels that are in the pending close
state. The ChainArbitrator maintains a set of ChannelArbitrators, one
for each channel that hasn’t yet been fully resolved.
Outside sub-systems should send new channels to the arbitrator once
they’ve opened. Additionally, they can also trigger manual
interventions to close out a channel on chain forcibly, or just to
signal that a channel has been closed cooperatively.
Finally, (for now) the ChainArbitrator should be notified once a fresh
set of signals for a channel becomes available. The ChannelArbitrator
for the channel will use these set of signals to be notified when an
on-chain event happens.
