lnwallet: partition state update logs within channel state machine

This commit patrons the state update logs properly within the channel
state machine. This change fixes a number of bugs caused by treating a
central log as two logically distinct logs. Rather than having a bit
indicating if the entry is incoming/outgoing, an entry is added to a
remote or local log depending on which modification method is used.

As a result the code is much easier to follow due to separation of
concerts.

Finally, when attempting to sign a new update with an exhausted
renovation window a distinct error is returned in order to allow higher
level callers to properly back-off and handle the protocol event.

lnwallet/log.go

@@ -54,3 +54,19 @@ func SetLogWriter(w io.Writer, level string) error {
 	UseLogger(l)
 	return nil
 }
+
+// logClosure is used to provide a closure over expensive logging operations
+// so don't have to be performed when the logging level doesn't warrant it.
+type logClosure func() string
+
+// String invokes the underlying function and returns the result.
+func (c logClosure) String() string {
+	return c()
+}
+
+// newLogClosure returns a new closure over a function that returns a string
+// which itself provides a Stringer interface so that it can be used with the
+// logging system.
+func newLogClosure(c func() string) logClosure {
+	return logClosure(c)
+}