p2p: fix Peer shutdown deadlocks

There were multiple synchronization issues in the disconnect handling,
all caused by the odd special-casing of Peer.readLoop errors. Remove the
special handling of read errors and make readLoop part of the Peer
WaitGroup.

Thanks to @Gustav-Simonsson for pointing at arrows in a diagram
and playing rubber-duck.

p2p/peer_error.go

@@ -98,15 +98,13 @@ func (d DiscReason) String() string {
 	return discReasonToString[d]
 }
 
-type discRequestedError DiscReason
-
-func (err discRequestedError) Error() string {
-	return fmt.Sprintf("disconnect requested: %v", DiscReason(err))
+func (d DiscReason) Error() string {
+	return d.String()
 }
 
 func discReasonForError(err error) DiscReason {
-	if reason, ok := err.(discRequestedError); ok {
-		return DiscReason(reason)
+	if reason, ok := err.(DiscReason); ok {
+		return reason
 	}
 	peerError, ok := err.(*peerError)
 	if !ok {