channeldb: re-design OpenChannel schema, update tests

The state of OpenChannel on disk has now been partitioned into several
buckets+keys within the db. At the top level, a set of prefixed keys
stored common data updated frequently (with every channel update).
These fields are stored at the top level in order to facilities prefix
scans, and to avoid read/write amplification due to
serialization/deserialization with each read/write.

Within the active channel bucket, a nested bucket keyed on the node’s
ID stores the remainder of the channel.

Additionally OpenChannel now uses elkrem rather than shachain, delivery
scripts instead of addresses, stores the total net fees, and splits the
csv delay into the remote vs local node’s.

Several TODO’s have been left lingering, to be visited in the near
future.

channeldb/db.go

@@ -132,3 +132,24 @@ func fileExists(path string) bool {
 	return true
 }
 
+// FetchOpenChannel...
+func (d *DB) FetchOpenChannel(nodeID [32]byte) (*OpenChannel, error) {
+	var channel *OpenChannel
+	err := d.store.View(func(tx *bolt.Tx) error {
+		// Get the bucket dedicated to storing the meta-data for open
+		// channels.
+		openChanBucket := tx.Bucket(openChannelBucket)
+		if openChannelBucket == nil {
+			return fmt.Errorf("open channel bucket does not exist")
+		}
+
+		oChannel, err := fetchOpenChannel(openChanBucket, nodeID, d.cryptoSystem)
+		if err != nil {
+			return err
+		}
+		channel = oChannel
+		return nil
+	})
+
+	return channel, err
+}