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.

lnwallet/script_utils.go

@@ -178,7 +178,7 @@ func senderHTLCScript(absoluteTimeout, relativeTimeout uint32, senderKey,
 	// Alternatively, the receiver can place a 0 as the second item of the
 	// witness stack if they wish to claim the HTLC with the proper
 	// pre-image as normal. In order to prevent an over-sized pre-image
-	// attack (which can create undesirable redemption asymmerties, we
+	// attack (which can create undesirable redemption asymmerties), we
 	// strongly require that all HTLC pre-images are exactly 32 bytes.
 	builder.AddOp(txscript.OP_ELSE)
 	builder.AddOp(txscript.OP_SIZE)