channeldb: implement commitment state update log

This commit implements a state update log which is intended the record
the relevant information for each state transition on disk. For each
state transition a delta should be written recording the new state. A
new method is also provided which is able to retrieve a previous
channel state based on a state update #.

At the moment no measures has been taken to optimize the space
utilization of each update on disk. There are several low-hanging
fruits which can be addressed at a later point. Ultimately the update
log itself should be implemented with an append-only flat file at the
storage level. In any case, the high level abstraction should be able
to maintained independent of differences in the on-disk format itself.

channeldb/error.go

@@ -7,4 +7,5 @@ var (
 
 	ErrNoActiveChannels = fmt.Errorf("no active channels exist")
 	ErrChannelNoExist   = fmt.Errorf("this channel does not exist")
+	ErrNoPastDeltas     = fmt.Errorf("channel has no recorded deltas")
 )