This commit adds PendingOpenChannel to SubscribeChannelEvents stream in
the gRPC API.
This is useful for keeping track of channel openings that Autopilot does.
It can also be used for the non-initator side of a channel opening to keep
track of channel openings.
This commit adds a channel event store to the channel fitness
package which is used to manage tracking of a node's channels.
It adds tracking for channel open/closed and peer online/offline
events for all channels that a node has open.
Events are consumed from channelNotifier and peerNotifier event
subscriptions. If either of these subscriptions is cancelled,
channel scoring stops, because both subscriptions are expected
to run until node shutdown.
Two functions are exposed to allow external callers to get uptime
information about a channel. GetLifespan returns the period over
which the channel has been monitored. GetUptime returns the channel's
uptime over a specified period. Callers can use these functions to
get the channel's remote peer uptime over its entire lifetime, or
a subset of that period.
This commit introduces the channel notifier which is a central source
of active, inactive, and closed channel events.
This notifier was originally intended to be used by the `SubscribeChannels`
streaming RPC call, but can be used by any subsystem that needs to be
notified on a channel becoming active, inactive or closed.
It may also be extended in the future to support other types of notifications.
