In this commit, the channelEventStore in the channel
fitness subsystem is changed to identify channels
by their outpoint rather than short channel id. This
change is made made becuase outpoints are the preferred
way to expose references over rpc, and easier to perform
queries within lnd.
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.
