channeldb: add FetchAllChannels method to return all active channels

This commit introduces a new method to channeldb: ‘FetchAllChannels’.
This method can be used to obtain the state of all active (currently
open) channels within the database. This method can be used for compute
basic channel-based metrics or exposed as an RPC in order to allow
clients to display/query channel data.

channeldb/nodes.go

@@ -49,6 +49,7 @@ type LinkNode struct {
 	// channel should be unilaterally closed due to inactivity.
 	//
 	// TODO(roasbeef): replace with block hash/height?
+	//  * possibly add a time-value metric into the heuristic?
 	LastSeen time.Time
 
 	// Addresses is a list of IP address in which either we were able to