autopilot: update the Node interface to return a raw bytes, not the key

In this commit, we modify the Node interface to return a set of raw
bytes, rather than the full pubkey struct. We do this as within the
package, commonly we only require the pubkey bytes for fingerprinting
purposes. Before this commit, we were forced to _always_ decompress the
pubkey which can be expensive done thousands of times a second.

autopilot/interface.go

@@ -17,8 +17,8 @@ import (
 type Node interface {
 	// PubKey is the identity public key of the node. This will be used to
 	// attempt to target a node for channel opening by the main autopilot
-	// agent.
-	PubKey() *btcec.PublicKey
+	// agent. The key will be returned in serialized compressed format.
+	PubKey() [33]byte
 
 	// Addrs returns a slice of publicly reachable public TCP addresses
 	// that the peer is known to be listening on.