autopilot: start threading contexts through

The `GraphSource` interface in the `autopilot` package is directly
implemented by the `graphdb.KVStore` and so we will eventually thread
contexts through to this interface. So in this commit, we start updating
the autopilot system to thread contexts through in preparation for
passing the context through to any calls made to the GraphSource.

Two context.TODOs are added here which will be addressed in follow up
commits.

autopilot/interface.go

@@ -1,6 +1,7 @@
 package autopilot
 
 import (
+	"context"
 	"net"
 
 	"github.com/btcsuite/btcd/btcec/v2"
@@ -35,7 +36,8 @@ type Node interface {
 	// iterate through all edges emanating from/to the target node. For
 	// each active channel, this function should be called with the
 	// populated ChannelEdge that describes the active channel.
-	ForEachChannel(func(ChannelEdge) error) error
+	ForEachChannel(context.Context, func(context.Context,
+		ChannelEdge) error) error
 }
 
 // LocalChannel is a simple struct which contains relevant details of a
@@ -83,7 +85,7 @@ type ChannelGraph interface {
 	// ForEachNode is a higher-order function that should be called once
 	// for each connected node within the channel graph. If the passed
 	// callback returns an error, then execution should be terminated.
-	ForEachNode(func(Node) error) error
+	ForEachNode(context.Context, func(context.Context, Node) error) error
 }
 
 // NodeScore is a tuple mapping a NodeID to a score indicating the preference