channeldb: add new ForwardingLog storage namespace

In this commit, we add a new storage namespace to channeldb: the
ForwardingLog. This log will be used by higher level sub-systems to log
each successfully completed HTLC. Each payment circuit will be
summarized as a “ForwardingEvent”. A series of events can then be
queried via a time slice query. In a time slice query, the caller
specifies a time range, a number of events to skip, and the max number
of events to return. Each query will return the index of the final
item. As we have a max number of events we’ll return in a response,
callers may need to use this last offset index to seek further by
skipping that number of entries. Combining these fields, callers are
able to query the time series, skipping an arbitrary amount of events,
and capping the max number of returned events.

channeldb/error.go

@@ -85,4 +85,8 @@ var (
 	// ErrNoClosedChannels is returned when a node is queries for all the
 	// channels it has closed, but it hasn't yet closed any channels.
 	ErrNoClosedChannels = fmt.Errorf("no channel have been closed yet")
+
+	// ErrNoForwardingEvents is returned in the case that a query fails due
+	// to the log not having any recorded events.
+	ErrNoForwardingEvents = fmt.Errorf("no recorded forwarding events")
 )