htlcswitch: recreate hlcswitch from scratch

This commit gives the start for making the htlc manager and htlc switch
testable. The testability of htlc switch have been achieved by mocking
all external subsystems. The concrete list of updates:

1. create standalone package for htlc switch.
2. add "ChannelLink" interface, which represent the previous htlc link.
3. add "Peer" interface, which represent the remote node inside our
subsystem.
4. add htlc switch config to htlc switch susbystem, which stores the
handlers which are not elongs to any of the above interfaces.

With this commit we are able test htlc switch even without having
the concrete implementation of Peer, ChannelLink structures, they will
be added later.

htlcswitch/packet.go

@@ -0,0 +1,78 @@
+package htlcswitch
+
+import (
+	"crypto/sha256"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/roasbeef/btcutil"
+)
+
+// htlcPacket is a wrapper around htlc lnwire update, which adds additional
+// information which is needed by this package.
+type htlcPacket struct {
+	// payHash payment hash of htlc request.
+	// NOTE: This fields is initialized only in settle and fail packets.
+	payHash [sha256.Size]byte
+
+	// dest is the next channel to which this update will be applied.
+	// TODO(andrew.shvv) use short channel id instead.
+	dest HopID
+
+	// src is a previous channel to which htlc was applied.
+	// TODO(andrew.shvv) use short channel id instead.
+	src lnwire.ChannelID
+
+	// htlc lnwire message type of which depends on switch request type.
+	htlc lnwire.Message
+
+	// TODO(andrew.shvv) should be removed after introducing sphinx payment.
+	amount btcutil.Amount
+}
+
+// newInitPacket creates htlc switch add packet which encapsulates the
+// add htlc request and additional information for proper forwarding over
+// htlc switch.
+func newInitPacket(dest HopID, htlc *lnwire.UpdateAddHTLC) *htlcPacket {
+	return &htlcPacket{
+		dest: dest,
+		htlc: htlc,
+	}
+}
+
+// newAddPacket creates htlc switch add packet which encapsulates the
+// add htlc request and additional information for proper forwarding over
+// htlc switch.
+func newAddPacket(src lnwire.ChannelID, dest HopID,
+	htlc *lnwire.UpdateAddHTLC) *htlcPacket {
+	return &htlcPacket{
+		dest: dest,
+		src:  src,
+		htlc: htlc,
+	}
+}
+
+// newSettlePacket creates htlc switch ack/settle packet which encapsulates the
+// settle htlc request which should be created and sent back by last hope in
+// htlc path.
+func newSettlePacket(src lnwire.ChannelID, htlc *lnwire.UpdateFufillHTLC,
+	payHash [sha256.Size]byte, amount btcutil.Amount) *htlcPacket {
+	return &htlcPacket{
+		src:     src,
+		payHash: payHash,
+		htlc:    htlc,
+		amount:  amount,
+	}
+}
+
+// newFailPacket creates htlc switch fail packet which encapsulates the fail
+// htlc request which propagated back to the original hope who sent the htlc
+// add request if something wrong happened on the path to the final destination.
+func newFailPacket(src lnwire.ChannelID, htlc *lnwire.UpdateFailHTLC,
+	payHash [sha256.Size]byte, amount btcutil.Amount) *htlcPacket {
+	return &htlcPacket{
+		src:     src,
+		payHash: payHash,
+		htlc:    htlc,
+		amount:  amount,
+	}
+}